Hey All,
Tons of great conversation on this, and Sarai's point is well made. But let's just take one total look at it from top to bottom.
Byron possessed powers of observation that would have made him a great anthropologist and that makes his letters as a group the rival of the best novels of the time.
We should notice right away that the split "makes/make" creates a subject-verb agreement split. From there, it's about modifiers.
(A) makes his letters as a group the rival of
PROBLEM: To check subject verb, we have to notice the parallelism marker "and". "that makes" wants to match with "that would have made". The subject of both is the same, "powers of observation", which is plural. We need a plural verb, so no "s".
(B) makes his letters as a group one to rival
PROBLEM: Same as above.The modifier here is probably fine.
(C) makes his letters a group rivaling
PROBLEM: Same as above. The modifier here would probably we fine.
(D) make his letters as a group the rival of
ANSWER: Correct subject-verb agreement. Now the modifier. What we have here is "letters" modified by the prepositional phrase "as a group", which is being compared to "the best novels of the time". Are "letters" comparable with "the best novels"? Yes.
(E) make his letters a group which is the rival of
PROBLEM: Correct subject-verb agreement. Sarai's point is great, which is that you cannot use "which" without a comma. However, it's still iffy, because we end up comparing "a group" to "the best novels", which is no good.
Hope that helps!
-t