Columnist: There are several symptoms of depression. One symptom is sleeping more than 12 hours a day. Therefore, you should buy an alarm clock so that you do not sleep more than 8 hours a day to cure your depression.

If the article also noted that excessive sleeping is caused by reduced serotonin levels resulting from depression, then which of the arguments below best describes the logical flaw in the columnist’s reasoning?
A. Sleep has many beneficial effects.
B. The columnist has mistaken an argument for an explanation.
C. The columnist has mistaken a premise for a conclusion.
D. The columnist has failed to consider that an alarm clock may not be a reliable means of assuring a person wakes up.
E. The columnist has mistaken a symptom for a cause.

(E) (A) is incorrect because the fact that sleep has beneficial effects does not address the potential problems resulting from too much sleep.

We can rule out (B) because the article did not make an argument for why people get depressed, nor did it explain the occurrence of depression; it merely listed behaviors associated with the illness.

Choice (C) can be eliminated for the same reason: premises and conclusions are elements of an argument.

(D) is incorrect because whether an alarm clock is a reliable means of waking a person up is outside the scope.

That leaves us with (E). The newspaper story indicated that depression causes oversleeping, not that oversleeping causes depression. Oversleeping is therefore an indicator – a symptom – of depression, which the columnist has mistaken for a cause. The answer must be (E).
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I don't understand E.

I think D is the most closest answer.

Choice (D) is incorrect because it mistakes a symptom for a cause. In other words, oversleeping does not cause depression; rather, it is depression that causes oversleeping. Therefore, eliminating oversleeping will not eliminate depression. Whether or not the alarm clock reliably prevents oversleeping (the symptom), it will not "cure" depression (the cause).

Think of this straightforward example: Sunshine causes trees to grow, but cutting down trees will not stop the sun from shining.