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.