Africa's black rhino population in the mid-1970's numbered about 20,00

Good questions, AXAN. Notice, first, that black rhino is used as an adjective to define a particular population, and in such a context, we use the singular noun form for that adjective. For instance, we say the human population, not the humans population. Thus, the sentence should not use their to refer to the black rhino as a species. Answer choice (D) would already be out of the running. Still, and to touch on some of your other questions, we cannot ignore more at the head of answer choices (C) through (E). Since ten times can function as a comparative without more, ten times more introduces a redundancy that is better avoided. Besides, the GMAT™ tightly governs elements in a comparison, and since the first element is population, so, too, should be the second. Follow the nouns to tell what is being compared. Answer choice (C) refers to an amount; (D) to black rhinos and an estimate in some sort of lopsided comparison; and (E) to that and, again, an amount. Finally, in answer choice (B), that should have a logical antecedent that we can substitute in, and here, we would be looking for a singular noun. The only such word I can find earlier in the sentence is population, and it makes no sense to say [the population] of their estimated population. You cannot substitute the number as a derivative of the earlier numbered just to patch things up. That is more restricted in use on this test. In short:

Perhaps the question makes more sense now. Comparisons are heavily tested, so they are worth careful study.

- Andrew