The sentence cuando juegas al fútbol, corres mucho means when you play soccer, you run a lot. So corres is the more logical answer because this comes from the verb correr that means to run and as you well know you run a lot when playing this sport. This sentence is given in the simple present tense, hence corres is the conjugation of correr that matches the second person singular in informal form (tú) in the simple present tense.
