Normally I've seen that sentences which start with आप end with हैं, but it's not the case here, why is that?
है is translated with पसंद here, meaning 'pleasing' (as a noun). So the sentence is sort of saying "To you, what is pleasing?" It's not 'you' that 'is', it's the 'pleasing', which is singular. Could a mother tonuge speaker double check my logic?
Not a native, but that seems to be almost correct. The subject here is the क्या, which can be either singular or plural. So I think हैं is valid here, but it would change the meaning - you would be expecting the answer to be something plural.