Answer:
¿Cómo está?
Explanation:
This is a common conversation in Spanish, to answer the exercise you need to know classic vocabulary about presentation or starting a conversation. The correct answer of the exercise is ''¿Cómo está?'' because you already know that the answer of the professor is ''estoy bien'', which means there was a question before asking how he was.
The exact translation of the exercise is:
Sergio: Hello, professor. How are you?
Professor: I'm fine. Thank you, Sergio.
