Right answer:
a. Sí, conocí a muchas
This is the more logical option because the question means Did you meet many people? and the answer means Yes, I met many people. First of all, you start the sentence with an affirmation using the word sí. Then, you give a response saying that in fact you met many people. Finally, the dialogue is as follows:
- ¿Conociste a muchas personas?
- Si, conocí a muchas.