(A) when Vassar was a small, recently founded women's college, founding professor and astronomer Maria Mitchell taught
as many astronomy majors in a given year
as there are today, when
(B) when Vassar was a small, recently founded women's college,
in a given year, founding professor and astronomer Maria Mitchell taught just as many astronomy majors as there are in a given year today, when
(C) while Vassar was a small, recently founded women's college, founding professor and astronomer Maria Mitchell taught a number of astronomy majors in a given year
such as there are today, when
(D) while Vassar was a small, recently founded women's college, founding professor and astronomer Maria Mitchell taught
such a number of astronomy majors in a given year as are there today, whereas
(E) when Vassar was a small, recently founded women's college, founding professor and astronomer Maria Mitchell taught a number of astronomy majors
just as large in a given year as the number that is there today, while
A is the best. Uses the correct Idiom and correct placement of the modifiers