Right answer:
camines
This sentence is given in the imperative mood. This mood is used to tell someone to do something in a direct manner. Put another way, sentences in the imperative mood are commands. So I am telling Pablo not to walk around the park because it's late. Thus, camines is the conjugation of the verb caminar that matches the second person singular in this mood. Finally:
Pablo, no camines por el parque, por favor. Es muy tarde.