Premise:
A researcher studying lawyers found that, on average, lawyers took more classes in philosophy as undergraduates than did members of other professions.

Conclusion:
The research surmised that students who take philosophy classes are more likely to become lawyers.

Assumption:
More philosophy classes lead to becoming a lawyer

Gap:
The writer of the argument fails to consider that students might have chosen Philosophy classes because students are advised to do so. The argument writer rather thinks that students will become lawyers because they are taking more philosophy classes.

So, if we attack the point "students will not become lawyers even though they are taking more philosophy classes" and disprove it, we will get our answer. We can disprove the argument by filling the gap.

So, the answer is option (A)