What part of speech is 'are'?

Are is an i nterrogative word. Meaning it's just a word for asking a question about something.

The word 'are' can be a verb or a noun depending on the context.

Verb

~In most cases 'are' is classified as a verb, more specifically as a linking verb. When used as a linking verb, it connects the subject with the other parts of the sentence that provides additional information about it.

For example,

The boys are playing in the playground.

Noun

~This word is categorized under noun, if it is to refer to a unit of measurement that is equivalent to a hundred square meters.

For example,

How many are in one acre?