Which sentence contains an elliptical adverb clause in which one or more words is understood? as our band took the stage, a hush fell over the crowd. i know how to play as many songs as he. we tuned up our instruments as the band before us finished their set.

The correct answer is - I know how to play as many songs as he.
An elliptical sentence means that there are some words which are not written in the sentence directly, but which are still understood nevertheless. In the sentence above, you know that the entire sentence is 'as many songs as he does,' however, you don't have to write the verb 'does' because the sentence is clear even without it.