Africa's black rhino population in the mid-1970's numbered about 20,000, ten times the estimated population of 2,000 in 1997.

(A) the estimated population
(B) that of their estimated population
(C) more than the population's estimated amount
(D) more black rhinos than their population estimate
(E) more than that of their population's estimated amount

Now look each option one by one.
D -> Has the wrond use of "Estimate". This word should be in some adjective format.
E -> Too much wordy.
C -> wrong use of estimated amount and population.
B -> "That" stands for what?
A -> Only the best option left.
Again, this type of SC is very common - if u refer any hand book for SC, u will get the answer.

camitava wrote:

Africa's black rhino population in the mid-1970's numbered about 20,000, ten times the estimated population of 2,000 in 1997.

(A) the estimated population
(B) that of their estimated population
(C) more than the population's estimated amount
(D) more black rhinos than their population estimate
(E) more than that of their population's estimated amount

Now look each option one by one.
D -> Has the wrond use of "Estimate". This word should be in some adjective format.

Estimate can also be used as a noun, rigiht?
Below are definition and examples from Longman dictionary.

estimate noun

a calculation of the value, size, amount etc of something

a rough estimate (=not an exact calculation) of how much time we'll need
A conservative estimate (=a deliberately low estimate) puts annual sales at around $100 million.
The figure will only be about two million, less than half the original estimate.
Some estimates put the number of deaths at several hundred.

Yah Kaijen I got ur point! But tell me one thing man in -

more black rhinos than their population estimate

if estimate is used as noun, then population and estimate both nouns are used together side by side! But in English it should not be. Right? At least a preposition is required. Got it kaijen what I want to explain?

Check for pronoun error. "Their" refers back to rhino population(singular). Therefore B, D and E eliminated. C uses amount to refer to rhino populaton-- grammatically incorrect. Thus option A

IMO:A