Answer :
The correct option is C, because it begins with an adverb. An adverb has "ly" at the end of the word, so it is surprisingly.
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".