"Which one is you?" vs "Which one are you?"

Solution 1:

• 1.) Which one is you?

• 2.) Which one are you?

Which is correct?

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Both are "correct". They just have different subjects.

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LONG ANSWER VERSION: Let's identify the subject of each interrogative clause, by using the verb's number as the indicator:

• 1.a) Which one is you?

• 2.a) Which one are you?

Notice that there is formal subject-verb agreement between the subject and verb in each version.

Let's verify that we've actually identified the subject, by looking at a related declarative clause for each of those above two interrogative clauses.

For #1.a. "Which one is you?":

• 1.b) That one is you. -- [declarative clause]
• 1.a) Which one is you? -- [interrogative clause]

The conversion from declarative #1.b to the interrogative #1.a was a straightforward change from "That one" to "Which one". The subject of both versions are before the verb.

For #2.a. "Which one are you?":

• 2.b) You are that one.
• 2.c) You are which one? -- [interrogative clause, interrogative phrase in situ]
• 2.a) Which one are you? -- [interrogative clause, fronted interrogative phrase]

Notice that when the interrogative phrase ("which one") is fronted in a main clause, that causes obligatory subject-auxiliary verb inversion. That is what happened between the last two steps.

To verify this, we can use a multi-word verb phrase to see this subject-auxilary verb inversion working:

• 3.a) You would be that one.

• 3.b) You would be which one?

• 3.c) Which one would you be?

Notice how the subject "you" ended up getting sandwiched between "would - be".

A similar exercise can be done with the subject "Which one", except there is no subject-auxiliary verb inversion because the interrogative phrase is the subject:

• 4.a) That one would be you.

• 4.b) Which one would be you?

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Short story: One way to find the subject of an interrogative main clause is to convert the example sentence into one that uses a multi-word verb phrase. The subject will either be in the front before the verb phrase, or else it will be sandwiched in between the verbs.

Using the OP's two examples:

• 1.) Which one is you?

• 2.) Which one are you?

we see that the versions can be converted into the below:

• 1.x) Which one will be you?

• 2.x) Which one will you be?

Since the OP's two examples only had a single verb in them ("is" vs "are"), then as to what the subject is will, in this case, depend on the verb that is used -- for the speaker/writer will have used subject-verb agreement between the subject and verb.

So, the answer is: Both are "correct". They just have different subjects.

Solution 2:

An illustrative example taken from Halliday, An Introduction to Functional Grammar, (2004)

You're taking part in a play; but I don’t know whether you are hero or villain. Here is our conversation:
Which are you? — Which am I? Oh, I’m the villain.
(Subject:I, Complement:the villain)

Next you show me a photograph of the cast all made up; the dialogue now goes:
Which is you? — Which is me? Oh, the ugly one is me.
(Subject:the ugly one, Complement:me)

We can also postulate that you don't yet know which role you'll play, so you ask the director...

Which am I? (which role am I playing?, which do I represent?)

Or you're so heavily made up you don't recognise yourself in the "cast photo", so you ask...

Which is me? (which image is me?, which represents me?)

Thus both versions are potentially correct, depending on context. We can't infer anything from the word you, because unlike I/me, the same pronoun form is used in both cases. But in OP's specific context ("Which image represents you?"), the correct form is obviously "Which is you?".

Solution 3:

The correct answer is 'Which one is you?'.

But you could say 'You are which one?'

If 'Which one' is the subject of the sentence the verb is 'is'. If 'you' is the subject the verb is 'are'.

Solution 4:

I have perviously treated both as correct, but in this situation I would prefer Which one is you?

Is

My reasoning for Which one is you? stems from the thought that you are asking the subject to identify themselves as a member of a set or group.

This can be compared to the saying a passenger might use when a driver pulls up to their house, "Well, this is me." (meaning this is my house). A better example might be when choosing objects - a waitress brings a tray full of plates and, upon spying the plate of chicken wings, you exclaim, "That one is mine!"

Are

Which one are you? is still a correct question, but it is better for allowing the subject to identify an attribute of themselves.

For example: A recent poll showed that 70% of Stack Exchange users are right handed, while 30% are left handed or ambidextrous. Which one are you?"