The correct sentence is "Who is the audience?" This is because "audience" is typically treated as a singular collective noun, referring to a group as a single entity. Therefore, it takes the singular verb "is."

Mehran is correct. Any good dictionary will confirm this. For example, The Oxford English Dictionary: audience (noun) A2 [countable + singular or plural verb] the group of people who have gathered to watch or listen to something (a play, concert, somebody speaking, etc.) The audience was/were clapping for 10 minutes. The debate was televised in front of a live audience. an audience of 10 000 Audience members voted on their favorite presentations. We encourage audience participation during our show.

Both options are correct. The choice between "Who is the audience?" and "Who are the audience?" depends on how you are conceptualizing the audience. "Who is the audience?" treats the audience as a singular entity, perhaps a collective group. This is often used when referring to the audience as a single, unified entity. "Who are the audience?" treats the audience as a plural group of individuals. This is more common when emphasizing the individual members of the audience. In most contexts, either option would be acceptable. However, if you want to emphasize the individuals within the audience, "Who are the audience?" would be more appropriate.

Just to add my two penn'orth... You can use both, but (as Mehran says) they mean different things. "Who are the audience?" - more or less synonymous with "who is in the audience?" I think this question is most likely to be used when you are talking about a literal audience (such as for a performance). The question invites an answer which describes in detail who has actually turned up. So the answer might be "40% old ladies, 25% schoolchildren and 35% cats." "Who is the audience?" - this is more likely to be asked with the modern meaning of "audience" meaning people who are intended to be engaged with some kind of product - eg. the audience for a book, or an event: "who is the audience for this open day?". It's likely to mean the intended audience, rather than the people who have actually engaged. The answer will be something like "people under the age of 40 with at least two children".

I'm pretty sure that "audience" is singular, #1 is correct, and #2 is incorrect. But I'm going to check a dictionary, just to be certain. I was right, although I missed one nuance. Here are the details for anyone curious. The two online dictionaries I checked don't address this directly. But all the definitions Merriam Webster all use singular grammar in the example, e.g. "a group of listeners or spectators: the concert attracted a large audience." Something I didn't think of--but it doesn't change my answer--is that there is a rare meaning in which a plural form of the word is possible. However, it is the regular plural form, "audiences." This is the meaning of "a formal meeting, hearing, or interview," as in "an audience with the Pope." But it 1) it doesn't fit with a "who" question, and 2) you would use it the plural form, "audiences." For example, it would be possible to say "The Pope's audiences are scheduled on Wednesdays."