A dependent clause

(A) expresses a complete thought

(B) leaves a thought unfinished

(C) is a Sentence

(D) contains a comma splice

I am really sure it is letter A

A dependent clause B. leaves a thought unfinished.

Dependent clauses are incomplete, meaning that they cannot stand on their own, but rather have to be included into larger, independent clauses. The remaining options all refer to independent clauses, and are thus incorrect.