Explanation

Reading the question: There are various pieces to this argument. The question stem orients us, because it tells us to "explain." That means that we'll need a "mystery" with two parts, and that the correct answer will touch on both parts.

Creating a filter: The two pieces of our mystery are 1) the percentage enrolled in the program hasn't increased, even though 2) the campaign appeared to work... Youngsters remembered the advertising. Further, the campaign convinced them of the importance to having a plan. Perhaps there is some other consideration that hasn't been included here, an overlooked problem? We'll look for an "overlooked problem" but above all expect the correct answer to touch on both 1) and 2).

Applying the filter: We'll go out of order. Choice (A) is not an overlooked problem and doesn't connect to 2); it doesn't address why the plan seemed to work. Choice (D) is not an overlooked problem and doesn't connect to 1) or 2). Choices (B), (C), and (E) are all pretty good options. Choice (B) presents an overlooked problem, and it connects to 1). However, it doesn't connect to 2); even if the plan is inconvenient, if the percentage who thought it was important went up, why didn't enrollment go up? They decided that it was important, so they should have done it anyway. So (B) is out. Choice (E) is similar to (B); it presents a disadvantage with the plan, hammering on 1), but fails to connect well to 2). For example, (E) says "most," not "all," so it suggests that some youngsters could afford the plan. Given that fact, even then the percentage enrolled should have gone up somewhat. Finally, choice (C) connects to both 1) and 2). The outreach could appear to work but fail to increase enrollment if the youngsters never learned how to enroll. The correct answer is (C).