GMAT Sentence Correction (SC) | Re: A professor at the university has taken a sabbatical to research

Although "he" is apparently ambiguous, there is a principle stated in Manhattan SC guide as follows:
"Proximity: The pronoun should refer to the closest eligible antecedent."
Nonetheless the guide also makes it clear that this principle is "not absolute".

Now in the subject question options A to D can be eliminated because of wrong idiomatic usage of "research". Option E is the best, although there is a slight ambiguity with the pronoun "he"