I think you are correct but the question asks to write what you hear, and te is to be heard.
"Pystyttekö te laulamaan?" is what you could ask if you wanted to know if the people in question are physically able to sing right now, or perhaps you want to know they're not too emotionally overwhelmed or something like that. "Osaatteko te laulaa?" would also translate to "Do you know how to sing?"
For example, "Do you know how to read?" = "Osaatko lukea?" But "Are you able to read without glasses?" = Pystytkö lukemaan ilman silmälaseja?"
"Are you able" and "can you" are really very interchageable in english. I thought osaatko asked "do you have the ability - (to do something)". Correct? As opposed to asking someone if they would be prepared to give us a tune.
"Can you.." could be used in exactly the same way. Either "do you have the ability" (=osaatko ?) Or "could you give us a tune" without more context both usages are correct, if my understanding of ossatko is right.