What part of speech answers the question why?

The part of speech that answers the question "why" is an adverb.

Adverbs often describe the reason, manner, or time of an action or state of being.

Here are some examples:

* Why did you go to the store? Because I needed milk. (Because is an adverb that answers "why")

* He ran quickly to catch the bus. (Quickly is an adverb that describes the manner of running)

* She arrived late for the meeting. (Late is an adverb that describes the time of arrival)

Let me know if you'd like more examples!