In the late 19th century, when Vassar was a small, recently founded wo

I think we can also approach the issue via another route. Let us first ask - Is the broad-spectrum contrasting conjunction – while - appropriate in the given case or should we use a narrow band and pin pointing – when - for the subordinate conjunction. The usage -while - is not trying to denote a contrast here but is indicating a time frame. Therefore - When - is the proper diction in the given case So Choices C and D can be dropped instantly.

E also has to be dropped with the same contempt, because it is not clear whether the comparison is between - a number -, a countable plural noun, with - the number -a singular noun or it is between - the large astronomy majors - with -the number. –

Now between A and B:

In B, the misplaced prepositional modifier, - in a given year - means to say that Vassar was a small, recently founded women's college, in a given year as if it became a different thing in other years. This is not the text’s intent. Hence let us drop B also

IMO, A stands out as the best of the lot, for using the right subordinate conjunction – when - for both the past and present time frames and for placing the prepositional modifier -in a given year - at its due place