The sentence that contains an elliptical adverb clause in which one or more words is understood is option A. I know how to play as many songs as he.
The reason why this sentence contains an elliptical adverb clause is that the verb "does" is supposed to come after he, but it's not written.
An elliptical clause is a type of dependent, or subordinate, clause that is missing a word or words. Usually, the missing part is a verb or verb phrase.