"The cow is his cow."
Translation:La mucca è la sua mucca.
Just be careful not to use "suo/sua" for "their," as it is not used for the plural in Italian (just "his," "her," or "your" [for the formal you]). It took me forever to get used to this, since I had learned Spanish before learning Italian! I don't think you mean to imply that from your sentence, but I wanted to clear up any potential confusion.
Just as Julian and Flex pointed out above, you can't really tell the gender when the sentence is given out of context like this. As a general rule, though, I try to remember that Latin-based languages are male-centric. You might have seen this already when you learned that ragazzi can = boys OR boys and girls. It has been a while since I did this one, but I believe that duolingo will accept either because there is no gender context in this sentence.
If this was a story, it might go something like: "Steve ha una mucca. La mucca è la sua mucca" and then you would already know to use "his" (as long as Steve gender identifies as male) in the translation. Granted, if the name is something like Sam or Elli, where the gender isn't necessarily discernible from the name alone, you'd have to make your best guess, wait for more information to come along in the paragraph, or ask for clarification. I hope that makes sense! I'm sorry, I'm not great at explaining this one.
I said: La mucca e la sua......wondering if I should have added..la sua mucca. the correct answer to me was...La mucca is la sua vacca. Come on now guys...this is the first I have seen vacca in our lessons...I also learned vacca for cow from my Italian family...but switched when Duolingo used mucca..but this time Duolingo the pulled a fast one on me!!!!