The part of speech that answer the question which one is a?

The part of speech that answers the question "Which one is a?" is an adjective.

Here's why:

* Adjectives describe nouns or pronouns. They tell us more about the quality, quantity, or characteristic of something.

For example:

* Which one is a blue car? (Blue describes the car)

* Which one is a large house? (Large describes the house)

* Which one is a fast runner? (Fast describes the runner)

The question "Which one is a?" is asking for a specific characteristic or quality to identify a particular item from a group. The answer to this question will be an adjective that describes the item.