Byron possessed powers of observation that would have made him a great

Hi Vaibhav & Ankit,

• 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.

Since both “that” refer to “powers of observation”, which is a plural entity, singular verb “makes” leads to SV number agreement error in the first three choices.

Now choosing the correct answer choice between D and E requires proper understanding of the intended meaning of the sentence. This sentence presents two things about Byron’s “powers of observation”. The first factor is not in the underlined portion of the sentence. So I will not get into the details of this.
The second bit says that because of his extra-ordinary powers of observation, Byron’s letters, as a group, are the rival of the best novels of that time. The intended meaning is that the letters are the rival of the best novels when they are put together in a group to give them the form of a novel.

Now let’s see what choice D is saying:
• Byron possessed powers of observation that would have made him a great anthropologist and that make his letters as a group the rival of the best novels of the time.
This sentence is absolutely correct because the SV agreement error that we found in the original sentence has been taken care of. Also, the sentence communicates the intended meaning. As a group, Byron’s letters are the rival of the best novels of his time.

Now let’s analyze Choice E:
• Byron possessed powers of observation that would have made him a great anthropologist and that make his letters a group which is the rival of the best novels of the time.
There is a shift in meaning in this choice. This choice now says that Byron’s powers of observation make the letters a group. This is not correct. Powers of observation does not make the letters a group. The sentence does not mean that. It does not say that letters become a group and then they rival the best novels of the time. The original sentence means that together as a group, the letters become the rival of the best novels of the time.

Hope this helps.
Thanks.
Shraddha