Final answer:
The word 'sabes' is the right choice to fill in the blank, meaning 'you know' in Spanish.
Step-by-step explanation:
The correct word to fill in the blank is sabes. The sentence translates to: 'You study every day and that's why you know biology well.' In this context, 'sabes' is the correct form of the verb 'saber' (to know) to match the subject 'tú' (you).
Learn more about Verb conjugation in Spanish