Answer:
The correct answer is you met.
Step-by-step explanation:
Conociste is a conjugation of the verb Conocer (to know) and is conjugated in Preterite tense, which indicates an action that was finished in the past.
Used in a sentence:
Tú conociste a Mark Ruffalo / You met Mark Ruffalo.
You heard translates to escuchaste.
You found out translates to descubriste.
You knew translates to supiste.
Conociste can also translate to you knew in some contexts, since the word has many meanings in Spanish, but I think you met is more correct.