Final answer:
The correct verb to fill in the blank is 'sabes'.
Step-by-step explanation:
The correct verb to fill in the blank in the given sentence is sabes.
The question is asking if you know how to play tennis. In Spanish, the verb 'saber' is used to express knowledge of facts or how to do something.
For example, you could say 'Sé jugar al tenis' to mean 'I know how to play tennis'.
Learn more about Spanish verb conjugation