The answer is A. you dont need semicollens for this sentence.
Is A because this is the answer
The word in italics of speech is and adverb. Option C is correct.
The first characteristic of the word to be identified as an advern is the suffic -ly.
In this particular case, the adverb thoroughly is describing the verb captivated. So that, this is another factor to take into account in order to identify an adverb and differ it from and adjective. Adverbs describe verbs while adjectives describe and works as modifiers of nouns.
Thoroughly means in a thorough manner.