I translated as "What drinks the cow?" which was marked wrong. The word order confuses me a bit here, though; isn't "kua" the object here, similar to how "vann" is the object in "Hvem drikker vann"?
To take a slightly different example; would "Hvem spiser kua" mean "Whom does the cow eat" or "Who is eating the cow"?
It's all about context here. It is impossible to drink a cow.
Therefore, "hva drikker kua" must translate to "what is the cow drinking/what does the cow drink."
"Hvem spiser kua" could mean either of the translations you listed above, but it makes most sense to translate it as "who is eating the cow" given how often one action happens versus the other.