The problem is with the English question 'Who is sneezing?' there is no indication that a plural answer is expected.
I'm wondering how I can tell this was plural? is there a singular response that would also be acceptable? Ki tüsszent?
Of course. "Ki tüsszent?" is very fine.
Judging by above comments plural was previously suggested and now it is singular.
Yes, your answer is the correct plural answer.
What is the difference between "ki" and "kik"?
You'll notice the plural form has the plural ending, -k.
In Hungarian, you can ask "Who?" questions using the plural form.
Yeah, that was what I originially thought.
So, in this example where we have "kik", is that because it is a question?
And could you use "ki" in questions?
Yes, it's because it's a question. "Ki" is perfectly acceptable too, just remember to change the verb to the singular form.
Ok, thank you very much for such a quick reply.