There is definitely a sufficient and necessary flaw going on here, but the answer choices provide us with a few of these options, so we have to dig deeper.
The argument makes the conclusion that any effective ad is humorous because effective ads convey their message, and humorous ads convey their message. The problem with this logic is that we cannot combine those two statements because conveying a message is a necessary condition for a humorous ad, but it's not a sufficient condition for one. Thus, we're looking for an answer choice that's saying that that the argument is assuming that only humorous ads convey their message.
(C) says, "It treats a necessary condition for an advertisement’s being effective as if it were a sufficient condition."
(C) is incorrect because in the final sentence of the argument, the condition that's presented as necessary for an advertisement being effective was that the ad must convey its message. However, this necessary condition WAS NOT treated by the argument in the stimulus as if it were a sufficient condition. Rather, the mistaken reversal that actually happened in the argument involved being humorous. Thus, we can get rid of this answer choice.
(A) says, "It takes for granted that nothing but humor can attract a person’s attention and hold it long enough for a message to be conveyed."
(A) is great because it's saying the argument is assuming that only humor can convey a message. This is what we were anticipating, and it points out the specific type of converse (sufficient and necessary) flaw that is occurring in the argument.
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