And in this case as well, KIK - plural, so why is it only "..is .." accepted, not "...are..."?
Probably because it's much less used than "who is". Report it so that it can be added as an alternative.
When used as a question word who is always singular in English. It can refer to plural subjects, but the verb is always conjugated singularly. When who is a relative pronoun - such as "The men who are sneezing are sick." - then it can be either singular or plural. It would be incorrect to say "Who are sneezing?".
Who are those people? -- question word with plural verb. Who is those people? is not possible.
But it may not be possible with verbs other than the copula.
I suppose there will always be a few exceptions. In this case, it sounds wrong to conjugate the verb as is because we've established the plurality of the subject. In most cases where number has not been explicitly stated who must be conjugated as a singular noun. But you do bring up a good point, I think the only way it is possible to make who as a question word plural is with copula verbs. I've never really though about this before so this is very interesting to think about. Let me know if you can think of anything that might disprove what I'm guessing, I'd love to figure this out definitively.
It looks like it depends on the transitiveness/transitivity (vel non) of the verb.
The correct answer in american English would be "who are the ones sneezing" or "Who is sneezing".
Hungarians try to avoid more than two consonants clashing together, so they will often insert vowels.
tüsszenteni - tüsszentek - tüsszentesz - tüsszent - tüsszentünk - tüsszentetek - tüsszentenek
And for low vowels: tart (to hold)
tartani - tartok - tartasz - tart - tartunk - tartotok - tartanak - tartalak (for the I-you form)
The problem is that you want to learn hungarian and duolingo is always correcting your english answers... Typos are not a problem in hungarian but not allowed in english!