Answer:
A
Explanation:
It's not a complete sentence. It leaves you hanging. It's only a <em>fragment</em> of the sentence. We don't know what Marshall did when he realized that Barney lied to him about the stain on the carpet.
Amy told us what she had done. The underline word What is an relative pronoun.
A relative pronoun is a pronoun that introduces dependent or relative clauses in a sentence.
Hope this helps :)
