Answer:
B. sepamos
Explanation:
We know that the answer must be in the nosotros form because of the use of "vamos" later in the sentence. Using this context, we can conclude that the entire sentence is using "nosotros", and the nosotros form of saber is sepamos.
Nora, what do you want to do this afternoon?
I'm not very good at Spanish so please do not make fun of me or hate on my answer