In order to EVALUATE an argument, we must first find the logical flaw in the argument.

This argument is conflating two different metrics:
1. the proportion of women among total students: (women students)/(women students + non-women students)
2. the proportion of all women of a certain age who *are* students: (women students)/(women students + women who are not students)

(Also note that this problem assumes a male/female gender binary, which itself can be considered logically flawed. But until the GMAT changes its stance on this, we'll have to treat these as mutually exclusive & comprehensive categories on questions like this).

Imagine that in 1959, there were 100,000 men and 100,000 women in these age ranges. (To keep things simple, let's not even worry about population increase over time).

Scenario A:
1959: 11% of women are enrolled --> 11,000. Let's imagine that 30,000 men are enrolled. That makes women a little over ~25% of the student population.
1981: 30% of women are enrolled --> 30,000. Let's say that the # of men stayed unchanged: 30,000 men. That makes women 50% of the student population.
The author's argument holds: the proportion of women out of all students has increased.

Scenario A:
1959: 11% of women are enrolled --> 11,000. Let's keep the same 30,000 men enrolled. Again, women = ~25% of the student population.
1981: 30% of women are enrolled --> 30,000. Let's now say that the number of men enrolled increases to 90,000. That makes women exactly 25% of the student population.
The proportion of women enrolled actually decreased in this scenario.

If we want to know about women as a proportion of all students, we also need to know whether the proportion of male students out of all male 18-25 yrs olds changed.

(A) The percentage of women between twenty and twenty-one who were not enrolled in college
This is already inferable from the given info.

(B) The percentage of women between twenty and twenty-five who graduated from college
Graduation is irrelevant to the argument - it's only dealing with enrollment proportions.

(C) The percentage of women who, after attending college, entered highly paid professions
Again, irrelevant.

(D) The percentage of men between twenty and twenty-one who were enrolled in college
Yes, this is exactly what's missing.

(E) The percentage of men who graduated from high school
Irrelevant to whether they go to college or not.

The answer is D.