Pretty simple flaw question here. Don't be fooled by the word "weakness" in the question stem. If it were a weaken question, it would ask us to add some extra piece of information to the argument; here we are simply describing the flaw through the answer choices.
Conclusion: college students do not write nearly as well as they used to.
Support: almost all of the papers that my students have done for me this year have been poorly written and ungrammatical.
Flaw: the author assumes that the work of his students is an accurate representation of the work of college students in general. He is over-generalizing from a small sample size.
A. the correct answer. Matches the flaw.
B. we don't need to challenge our premise, as this answer seems to be doing.
C. it dosent need to. Out of scope.
D. this is not a flaw. Argument don't need to always present contrary evidence.
E. this is not an issue here.