[求助]Prep-Section 4-2

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.
(A) makes his letters as a group the rival of
(B) makes his letters as a group one to rival
(C) makes his letters a group rivaling
(D) make his letters as a group the rival of
(E) make his letters a group which is the rival of

答案是D。这个as是怎么回事啊？

as a group 修饰letters