Conclusion:

- District 1 better at encouraging girls to pursue science than is District 2


Premises:

- In District 1 80% of schools win "Women in Science" award; In District 2 63% of schools win award

- Win award if at least 10 female students participate in science exposition

One could make many assumptions here, but the biggest, so far as I can see, is that there isn't some other factor, other than quality of science education, that accounts for this discrepancy.

A) First, we don't necessarily know that having more free time would make students more likely to pursue science. Second, if the students in District 1 had more free time than the students in district 2, that would undermine the argument's conclusion that the reason more students in District 1 pursue science is that the schools are better at encouraging such endeavors. Maybe District 2 is great, but the students there are just too busy. No good.

B) Overall population is outside the scope. This tells us nothing about the school demographics.

C) First, we can't assume that District 2 isn't bigger. It might be. Moreover, if we negate this one, it actually strengthens the conclusion. If there were more students in District 2, and District 1 still had more girls pursuing science, than it stands to reason that District 1 really is better at encouraging females students to do science. When we negate, we want the answer to weaken the conclusion, not strengthen it. This is out.

D) Now we're talking. Notice that this is the first answer choice that even references gender. That's what the whole argument is about! Now imagine we negate this answer. The negation would be: District 1 has significantly fewer all-male schools than District 2. Well, this would utterly destroy the conclusion. Consider a simple scenario. There are 100 schools in each district. This means that in District 1, 80 schools won the "Women in Science" award and 20 didn't. And in District 2 63 schools won the award and 37 didn't. Say that District 1 has no all-male schools and District 2 has 37. Well, in District 1, only 80% of the mixed-gender schools won the "Women in Science" award, but in District 2, if we remove the all-male schools, every single mixed gender school won the award! But the whole argument is that District 1 is better at encouraging science! If District 1 has fewer all-male schools, the reason more schools are winning the "Women in Science" award could be simply that more schools actually have female students not that this district is any better at encouraging its female students to pursue science. Because the negation destroys the argument, we know we have our answer.

E) This is just silly. This suggests nothing about District 1 vs District 2.

Answer is D