Thanks for the question! First, a side note for other readers of the LRB: this post refers to a page number in an older version of the LRB. In the current version, this question is on page 347
Ok, that aside, let's look at your question. As I discuss in the text, answers (A) and (C) are both similar in how they operate, so it's not surprising that if one attracted you, then the other did as well
In this argument, however, these two answers aren't assumptions because they address an issue that is outside the relationship being discussed in the argument.
In the argument, the author concludes that depression causes glasses to be worn (which I diagrammed out as Depression
Glasses ). At that point, the author is concluding that nothing else causes the the wearing of glasses, that the info is correct, and that it's not reversed. Answer choice (E) helps out because it confirms that the relationship isn't reversed (in other words, that the wearing of glasses doesn't cause depression).
But what about (A) and (C)? Let's look at (A), which states that depression isn't an organic condition of the body. Is that something the author assumes? No. The author concludes that depression causes the wearing of glasses, but the author never addresses what caused the depression in the first place. From the author's statements, it doesn't matter if it's an organic condition or not; the author is only worried about what happens once someone has
depression (they wear glasses) and not what caused the depression in the first place.
With (C), we have the same type of issue, although it is broader. (C) states that depression has a lot of causes, including some that are reasonable. But again, the author isn't addressing why people became depressed in the first place; the author is concerned about what happened once they were
To analogize, it's like a friend of your making an argument that war causes people to die. Answer choice (A) would say something like, "War is sometimes caused by natural conditions in a country." Is that a point assumed by your friend? No way to know, right? For (C), the answer would be that "War is caused by many things, some of which are reasonable." Is that an assumption of your friend's argument? No, because your friend is just talking about how war kills people, not what causes war. In the same way, (A) and (C) address issues that are "before" or prior to what the argument is about.
Please let me know if that helps. Thanks!