"Yes, I can."
Translation:Sì, io posso.
si, posso should be acceptable, I think.
si, sono posso would be redundant. Since potere expresses ability, "posso" could mean "I can" or "I am able". Adding sono would not really make sense.
Because it is wrong in conjugation. Correct is: io posso, tu puoi, lui/lei puo, noi possiamo, voi potete, loro possono.
I put "Si, io posso" and it was marked wrong, the correct answer given looked like "Si, io posso" with the Si, underlined. I can't see why mine was incorrect.
"Sì" is marked with a grave accent. Not without accent, nor with an acute one.