Final answer:
The verb saber should be conjugated as 'sepan'.
Step-by-step explanation:
The correct form of the verb saber in this sentence is sepan.
When used in the context of possibility or doubt, the verb saber is conjugated as sepas for tú, sepa for él/ella/usted, sepamos for nosotros/as, and sepan for ellos/ellas/ustedes.
For example, the sentence 'Es posible que tú sepas el vocabulario' means 'It is possible that you know the vocabulary'.
Learn more about Conjugation of the verb saber in Spanish