This is the wrong flaw. (A) takes a conditional statement and erroneously assumes that it’s still valid when reversed. By contrast, the stimulus takes a conditional statement and mistakenly assumes that it’s still valid when negated.

This is the wrong flaw. (B) is close to the valid contrapositive (if no balloons, then not Jenny’s birthday party). But it misapplies it: just because it isn’t her birthday party yet doesn’t mean it’s not her birthday. By contrast, the stimulus mistakenly assumes a conditional statement is still valid when negated.

This is the wrong flaw. Like (A), (C) takes a conditional statement and erroneously assumes that it’s still valid when reversed. By contrast, the stimulus takes a conditional statement and mistakenly assumes that it’s still valid when negated.

The author identifies a conditional relationship (if late flight, then miss meeting). He negates the sufficient condition (flight on time), and assumes the necessary condition is also negated (meeting not missed).

This is the same flaw we saw in the stimulus: negating a sufficient condition does not automatically justify negating a necessary condition.

This is a valid argument, unlike the stimulus. The conclusion of (E) is the contrapositive of the initial statement: the conditions are switched and negated. This is valid, as opposed to merely switching or merely negating the conditions. (Merely negating them is the flaw in the stimulus and (D), the correct answer choice.)