I pick C. "they", "it" and "there" seem ambiguous.

answer choice C here as well. A is so wrong it's not even funny ;]

Agree with C
Proper usage of conjunction "and" along with verb tense is what is tested here.

When you are combining sentences after you use a conjunction you need a pronoun that correctly refer to the anticedent which in our case happens to be the subject. Only choice C satisfies all these conditions.

The answer is def C. I saw this EXACT question in the GMAT prep software.

E can not be.

"and there will be an estimated...... and surpass China....." It is not parallel

It must be C

C is the best.

D & E out as sentence starting with there are not prefered in GMAT.
A Estimated as is not the correct idiom
B they ---pronoun mismatch

we are not comparing with population of india.

they is ambigous in B.
inA. It is refering to population of india, which can not have 1.6million population

In D and E there is ambigous.

So C is the only right choice.

Strongly support for C.

"The country" is necessary due to the appearance of the phrase "the world’s most populous nation" at the end of the sentence

The population of India has been steadily increasing for decades, and it will probably have what is estimated as 1.6 billion people by 2050 and surpass China as the world’s most populous nation.

A. it will probably have what is estimated as probably have and estimated ..either of them is redundant, estimated as is incorrect
B. they are likely to have they has no antecedent
C. the country will probably have correct...the country is used stylishly to ref to India
D. there will probably be there makes the sentence awkward
E. there will be an estimatesame as D
thats my 2 cents
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we need subject for "surpass" , so, E and D are gone because there is no such subject.
in B, "they" has no antecedent. gone.
gmat also test logic. in a choice if there is no grammatical mistake, we find logic mistake. logic mistake is so simple that we do not think gmat test this kind of error. but logic test is critical because we need to write logic sentence.
in A, "it" refer to " population". "population' can not have 1.6 bilion logically. we say " population is 1.6 bilion". A is gone.

I think that the point is this: given that B,D,E are wrong for pronouns mistakes, A and C are grammatically correct in fact in A it refers without doubts to the population of India.

The population of India has been steadily increasing for decades, and it will probably have what is estimated as 1.6 billion people by 2050 and surpass China as the world™s most populous nation. the population of india is compared with china--> eliminate A

The population of India has been steadily increasing for decades, and it will probably have what is estimated as 1.6 billion people by 2050 and surpass China as the world’s most populous nation.

A. it will probably have what is estimated as - is redundant (there's no reason to use both "probably" and "estimated"). additionally, "what is" doesn't make any sense.
B. they are likely to have - Pronoun they does not have plural antecedent here
C. the country will probably have - Correct
D. there will probably be - there is no actual indication that the 1.6 billion people will be in india. those choices just say "there will be", which is an idiomatic expression that doesn't indicate a place.
E. there will be an estimated - same as D

Answer C

takeaway: "there is/are" / "there was/were" / "there will be" / etc. DOES NOT INDICATE A PHYSICAL LOCATION. unlike most other uses of "there", in which "there" actually stands for some place, this "there" doesn't stand for anything.

if you want to use "there is" in reference to a particular location, you have to state the location, explicitly, elsewhere in the sentence.
ex: there are three cars in the driveway.
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This question is based on Construction.

In Option A, the pronoun ‘it’ refers to the population of India. The meaning conveyed by the usage of the pronoun in this option is that the population of India will probably have what is estimated as 1.6 billion people. Since this meaning is illogical, Option A can be eliminated.

In Option B also, the pronoun ‘they’ refers to the population of India. Besides being illogical, the use of the pronoun also creates a pronoun-antecedent disagreement, since the plural pronoun ‘they’ cannot refer to the singular antecedent ‘the population’. So, Option B can also be ruled out.

Option C has the appropriate construction. The noun ‘the country’ is mentioned in the option to convey the intended meaning that it is the country that will have the estimated population of 1.6 billion people. So, C is the best of all options.

Options D and E begin with the adverb ‘there’. The meaning conveyed by the usage of the adverb is that there will be an estimated 1.6 billion people and surpass China. The adverb ‘there’ refers to a place in which an action takes place. However, in neither of the options is the place mentioned, so we do not know what will surpass China. So, Options D and E can be eliminated.

Therefore, C is the most appropriate option.

Jayanthi Kumar.
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DE are not parallel structure: there xxx and surpass..
AB: Pronoun singular/plural, ambiguity issue.