The answer to your question would be that the sentence that begins with an adverb is the following one: Surprisingly, the flute is not as difficult to play as a I once thought. That is, the correct option would be C.
An adverb is a word or phrase that modifies or qualifies an adjective, verb, or another adverb, expressing a relation of time, place, manner, circumstance, etc. In this case, the adverb is "surprisingly".