I am guessing you mean which part of the sentence is incorrect, so that is what those alphabetic options represent.
score: Source
a large number of something.
"he sent scores of enthusiastic letters to friends"
It sounds wrong, and is wrong to assign a particular number before the word scores. If you want to say it your way, you need to reframe the sentence as:
There are scores of books which are lying unused in the library. (many number of books)
set: Source
a group or collection of things that belong together or resemble one another or are usually found together.
"a set of false teeth"
When saying sets of books, we determinedly mean there are only two distinct or repeated set of books that lie on the table.
There are two sets of books which are lying unused in the library. (only two sets of various books, no more, no less)