You could say "Hva spiser en ulv", but though grammatically correct, it sounds awkward in that context. It would be more common to say "Hvilket dyr (=what animal) spiser en ulv/ulver", and the plural "ulver" would most likely used instead of the singular, unless you're staring at a dead wolf in front of you.
Reading the comments, I'm glad it wasn't just me lol. My Dutch brain entered "What eats a wolf?" as answer and it was correct. Only later on I realised I also could have answered "What does a wolf eat?"
I love how this can be ambiguous in Norwegian and Dutch but not in English.