Answer:
The word when is being used as an adverb (C)
Explanation:
Here, when acts as an Interrogative adverb since it is used to ask a question, more specifically about time-related matters. Very often, interrogative adverbs are placed at the beginning of the question, and are followed by an auxiliary verb such as will, be, do and have, as we see in the given sentence.
Other examples of interrogative adverbs are why, how, how long, how many, and where.
