Official Explanation:

(A) Since cater comes before not only, it must apply to whatever comes after but also as well. So, it doesn’t make sense to put help after but also because you can’t cater and help. Conducting research needs to be followed by the preposition on. Conducting research is not parallel to conservation of. Finally, the correct idiom is help to do and not help in doing.

(B) Since cater comes before not only, it must apply to whatever comes after but also as well. So, it doesn’t make sense to put aid after but also because you can’t cater and aid. Conducting research needs to be followed by the preposition on.

(C) The correct answer.

(D) The plural zoos does not agree with the singular caters. Conducting research needs to be followed by the preposition on.

(E) The correct idioms are aid in conducting or help to conduct; this one incorrectly combines the two. Also conservation of should be preceded by the preposition in (in the conservation of).
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My answer is (C). It took me 41 seconds.

The key to this question is to maintain parallelism in "not only X...but also Y".

(A) "not only to the recreational needs of people” (not only + prepositional phrase) is not parallel with "but also help in conducting research and conservation" (but also + verb + prepositional phrase).

(B) "not only to the recreational needs of people” (not only + prepositional phrase) is not parallel with "but also aid in conducting research and in conservation" (but also + verb + prepositional phrase).

(D) SVA: Subject is plural.

(E) This option is tricky. "Verb Cater + preposition to + Noun phrase" and "Verb Help + preposition in + noun phrase" are actually parallel. But, we should prefer verb (conserve in c) to noun (conservation of in e). That is why I eliminate E.

Modern zoos cater not only to the recreational needs of people, but also help in conducting research and conservation of wild animals.


(A) cater not only to the recreational needs of people, but also help in conducting research and conservation of
cater not only .. but also help.. not ||

(B) cater not only to the recreational needs of people, but also aid in conducting research and in conservation of
cater not only ... but also aid .. not ||

(C) not only cater to the recreational needs of people, but also help to conduct research on and to conserve
Correct pairing of not only cater to ... but also help to...
And, correct SV agreement.

(D) not only caters to the recreational needs of people, but also aid in conducting research and conservation of
Zoos is plural. Thus, 'caters' should be 'cater'.

(E) not only cater to the recreational needs of people, but also help in conducting research on and conservation of
not only cater to .. but also help in..
'to' and 'in' need to be ||. (in this case, the same word)

Modern zoos cater not only to the recreational needs of people, but also help in conducting research and conservation of wild animals.
The first split is with cater inside the parallel market or outside the parallel marker.
if we try to understand the meaning by removing not only... (first arm), then it is something
zoos cater help/aid, a bit redundant?


Incorrect because of above-stated reason


same as A


SV agreement. zoos (plural) need plural verb, i.e. cater.
D is incorrect.


parallel, to the recreational needs, to conduct research, and to conserve
Correct


C is a better choice than E. I find a bit logical gap for "conservation of". To+V here shows a motive.
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Modern zoos cater not only to the recreational needs of people, but also help in conducting research and conservation of wild animals.

Idiom -> if Not only x but also y, then x and y must be in parallel

(A) cater not only to the recreational needs of people, but also help in conducting research and conservation of -> Not only "to the recreational..." but also " help in conducting research...". X and Y are not parallel. Incorrect.

(B) cater not only to the recreational needs of people, but also aid in conducting research and in conservation of -> Not only "to the recreational..." but also " aid in conducting research...". X and Y are not parallel. Incorrect.

(C) not only cater to the recreational needs of people, but also help to conduct research on and to conserve -> Not only "cater to the recreational needs..." but also " help to conduct research...". X and Y are parallel. Further, "to conduct" and "to conserve" are parallel too. Let's keep it.

(D) not only caters to the recreational needs of people, but also aid in conducting research and conservation of -> Not only "caters to the recreational needs ..." but also " aid in conducting research...". X and Y are not parallel. Incorrect.

(E) not only cater to the recreational needs of people, but also help in conducting research on and conservation of -> Not only "cater to the recreational needs ..." but also " help in conducting research...". X and Y are parallel. But, "in conducting" and "conservation of" are not parallel. Incorrect.

So, I think C.

Ans C

Analysis:
Not only X but also Y - A quick glance will tell us that the question tests the parallelism with this structure.

- cater to recreational needs of people
- help in conducting research and conserve animals.

The first underlined word cater is not common to both X and Y in the structure Not only X but also Y. so 'cater' should be inside in the format Not only cater but also Y

Eliminate Answer choices A and B based on parallelism error.

Modern Zoos - Here zoos is plural, so the use of caters is incorrect. Option D does this. Eliminate Option D for SV error.

In the second list , help in conducting research and conservation of is not parallel in E. We can eliminate E based on parallelism error.

Option C is our answer.
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Modern zoos cater not only to the recreational needs of people, but also help in conducting research and conservation of wild animals.


(A) cater not only to the recreational needs of people, but also help in conducting research and conservation of

(B) cater not only to the recreational needs of people, but also aid in conducting research and in conservation of

(C) not only cater to the recreational needs of people, but also help to conduct research on and to conserve

(D) not only caters to the recreational needs of people, but also aid in conducting research and conservation of

(E) not only cater to the recreational needs of people, but also help in conducting research on and conservation of

not only cater but also help is required to correct the sentence.

I will go with help in vs. help to considering meaning of sentence.
Here help is used as verb. so help to.... vs. help in conducting.... and help in conservation....is preferable. not sure-taking a chance

IMO E
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Modern zoos cater not only to the recreational needs of people, but also help in conducting research and conservation of wild animals.


(A) cater not only to the recreational needs of people, but also help in conducting research and conservation of Incorrect

parallelism error - .................not only to the ................ , but also help..............

(B) cater not only to the recreational needs of people, but also aid in conducting research and in conservation of Incorrect

parallelism error - .................not only to the ................ , but also aid..............


(C) not only cater to the recreational needs of people, but also help to conduct research on and to conserve Correct

(D) not only caters to the recreational needs of people, but also aid in conducting research and conservation of Incorrect

parallelism error - .................not only caters ................ , but also aid..............

(E) not only cater to the recreational needs of people, but also help in conducting research on and conservation of Incorrect

help in wrong