Hey Kristina,
Answer choice (D) cannot be correct becuase the stimulus tells us that if Joan gets an A

no need to do the presentation.
Therefore, as you correctly stated, the contrapositive would be:
need to present
Joan gets an A
That's exactly what answer choice (D) says, so that's not the flaw.
On the other hand, answer choice (B) suggests that just because an A was definitely good enough to excuse Joan from the class presentation doesn't mean that she couldn't have
also been excused with a lower grade. Say Joan got a B on her term paper - maybe that was also high enough to excuse her from the presentation.
Answer choice (B) points out the author's flawed logic, which incorrectly says
if Joan gets an A

no need to do the presentation, so
Joan gets an A 
Joan needs to do the presentation.
Do you see how that's incorrect logic?