A land-use planning institute in the Great Plains cites

C

1. 'they' incorrectly refers to singular 'institute'. C,D,E remain
2. D changes the meaning of the sentence. in A 'as major reasons to set aside the area...' implies that this recommendation is made by the institution, but in D 'that is the major reason to set aside the area' may imply author's recommendation....
3. C vs E - one reason vs. two reasons. I don't know a rule but one reason is better for me.

Interesting SC . Confused between C & E . Will pick C .

"Sharp declines" is one reason .

E

Population and prosperity - plural?
Institute - singular

Pronoun issue: A land-use planning institute is singular and requires singular pronoun.
Eliminate A and B

(A) as major reasons to set aside the area they call
(B) that are the major reasons to set aside the area they call
(C) as the major reason to set aside the area it calls [“As” subordinates a clause – “the major reason to set aside the area it calls the Plains Commons reserve” – hold it]
(D) that is the major reason to set aside the area called [ “that” modifies population and prosperity and so requires plural “are” – eliminate it]
(E) as major reasons to set aside the area it calls [ A clause cannot have two verbs - require “reason” as noun form not verb – eliminate it]

Answer: C

Are you sure there is 'delines' in the original question not 'decline'.
I am saying this for two reasons:
1. If there is 'declines' then no question abt it there ought be 'reasons' ( mind you reason or reasons has nothing to do with population and prosperiety) So E seems fine but I agree with
hanumayamma that a clause can not have two verbs..so E has a problem.

2.English is not my native tounge but I can say that I have never heard the use of word 'declines'...it has always been 'decline'..

Anyway if I am forced to choose one, it will be C

E. Plural pronoun reference for plural noun.

Literally sharp declines is plural, however, in this context its used as a singular whole.

C for me.

OA ?

After thinking of dilemma between C and E, I change my final choice to E. I think C changes the meaning of the sentence.

can 'decline in population' be a reason along?

The original sentence implies that each of the declines can be a reason along, but C says that only declines together can be a reason.

A land-use planning institute in the Great Plains cites sharp declines in population and prosperity, marked by a 50-percent decrease in the population of some cities and by an equivalent decrease in the median income throughout the region, as major reasons to set aside the area they call the Plains Commons reserve.

(A) as major reasons to set aside the area they call
(B) that are the major reasons to set aside the area they call
(C) as the major reason to set aside the area it calls
(D) that is the major reason to set aside the area called: D change the meaning of sentence.
(E) as major reasons to set aside the area it calls

OA is E.
THEY need to be replaced with IT. "declines in population and prosperity" shows that there is a plural noun, so REASONS is best.
CITES ... AS is idiom.

E for me.

A land-use planning institute in the Great Plains cites sharp declines in population and prosperity, marked by a 50-percent decrease in the population of some cities and by an equivalent decrease in the median income throughout the region, as major reasons to set aside the area it calls the Plains Commons reserve.

You can ignore the middle part and answer this question.

Yes, this is the same strategy when I deal with ....,xxx,....