Answer:
Caminar is correct!
Step-by-step explanation:
As you can see, the verb before that (prefiere), is already conjugated. I'm pretty sure two conjugated verbs cannot be right next to each other in every case. La niña prefiere (the girl prefers) would make the most sense with caminar (to walk) rather than camina (she/he walks), camino (I walk), and cominando (is walking).