AS PER KAPLAN:Read the Original Sentence Carefully, Looking for Errors:
The rule on the GMAT is that "who" refers to people and "which" refers to animals and things. For example, you would say, "I don't know who delivered the packages," but "Of all these packages, I don't know which is for me." This sentence is talking about pandas and should say "which," not "who." (Incidentally, if this sentence were about people, the correct word to use would be "whom," not "who," because it is the object of the preposition "of.")
Scan and Group the Answer Choices:
There is a useful 2-2-1 split among (A), which uses "who"; (B) and (D), which use "that"; and (C) and (E), which use "which."
Eliminate Wrong Answer Choices:
(A) is wrong for the reasons discussed above.
To decide between "that" and "which," remember that "that" is used when the information after it is needed to define the subject, while "which" is used when the information after it is not needed to define the subject. Here, the subject of the sentence is simply pandas in captivity, of which the two in the National Zoo are just examples. The sentence isn't about these two pandas. Thus, the correct word to use is "which," not "that," and both (B) and (D) can be eliminated.
(B) is also wrong because the word "and" suggests that the pandas at the National Zoo are being discussed in addition to "pandas in captivity," which makes no sense. (D) is also wrong because its use of "those" suggests that all the pandas in captivity are a subset of two within some other population (of animals?) in the National Zoo.
(C) uses "which," but it also uses the word "including." This is unnecessary, since the phrase "of which" already makes it clear that the two pandas at the National Zoo are a subset of the larger group. This unnecessary word makes (C) incorrect.
(E) is correct. It replaces "of who" with "of which" and doesn't insert an unnecessary words or phrases. It fits perfectly into the original sentence.
TAKEAWAY: The GMAT is strict about the use of "who." People only—no animals or things!