The OA is C. Here is the explanation:
In the original sentence, “much” incorrectly references the quantity of female graduate students. Students are countable, so “many” is the correct term. Additionally, “as 1981” incorrectly completes the comparison, illogically comparing the number of people (the “198,113 female science and engineering graduate students”) to a year (“1981”).
(A) This choice is incorrect as it repeats the original sentence.
(B) “As 1981” incorrectly completes the comparison, illogically comparing the number of people (the “198,113 female science and engineering graduate students”) to a year (“1981”).
(C) CORRECT. “Double the figure” places the emphasis on the number of female graduate students, and correctly completes the comparison between the number of people in one year (198,133 in 2003) and the number of people in another year (the figure for 1981).
(D) “What it was” is wordy, awkward, and unclear. The singular pronoun "it" has no clear antecedent.
(E) “Double that of 1981’s” is wordy, awkward, and unclear. The singular pronoun "that" has no clear antecedent. The possessive "1981's" is not followed by a noun to possess.