juegan would be used in a question, for example "que deporte juegan en la escuela?" (what sport do you play at school?)
jugamos would be used to answer the question "nosotros jugamos futbol en la escuela." (we play soccer at school.)
juega would be used to refer what somebody else does "el juega futbol en la escula." ( he plays soccer at school.)
so the correct answer would be jugar