"The baby is washing his hands"
There is no specific match. One says the baby washes his hands but the Swahilu only says the baby washes. I think it us meant to confuse us. The baby washes is the best answer but not as specific as the English sentence given
I think the Swahili sentence is intermediate in specificity between "The baby washes" and "The baby washes his/her hands". Kunawa specifies washing a part of ones own body, eg. face, hands, and I think hands might be the default interpretation. You could also say Mtoto ananawa mikono.
I tried "Mtoto ananawa mikono" and it was marked as incorrect. It wanted the "yake".