Imagine I'm looking at a photo containing a number of people's faces and I can't tell which one belongs to a certain friend of mine. I could ask him one of two things:
"Which one is you?"
"Which one are you?"
Which is correct?
Both are "correct". They just have different subjects.
LONG ANSWER VERSION: Let's identify the subject of each interrogative clause, by using the verb's number as the indicator:
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?":
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?":
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:
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:
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:
we see that the versions can be converted into the below:
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.
An illustrative example taken from Halliday, An Introduction to Functional Grammar, (2004)
We can also postulate that you don't yet know which role you'll play, so you ask the director...
Or you're so heavily made up you don't recognise yourself in the "cast photo", so you ask...
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?".
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'.
I have perviously treated both as correct, but in this situation I would prefer Which one is you?
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!"
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?"
I agree with asfallows on the preference of "Which one is you?" and a willingness to accept either.
Filling in the unsaid parts of the sentence:
Thank you for your interest in this question.
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