Which words in the sentence are the complete noun clause?

The two sisters looked for what fell behind the couch.

A.) The two sisters

B.) behind the couch

C.) the two sisters looked for what

D.) what fell behind the couch

On the choices provided the complete noun clause is letter D which is "what fell behind the couch" because it serves the role of the noun. The noun clause can not stand alone as a complete thought when your read the clause " what fell behind the couch" you will under stand it unless you read the entire sentence.