Answer:
The word <em>when</em> is being used as an adverb (C)
Explanation:
Here, <em>when</em> 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 <em><u>will</u></em><em>, be, do </em>and <em>have</em>, as we see in the given sentence.
Other examples of interrogative adverbs are <em>why, how, how long, how many, and where. </em>
