¿Cuándo vas a la playa? means "When are you going to the beach?"
Let's elimante the answers that aren't a possibility:
1. Generalmente estoy aquí.
2. Generalmente voy los domingos.
3. Generalmente estoy enfermo.
4. Generalmente van los martes.
We know that this question was asked to you, so the answer can't be something that refers to they. #4 uses "van" instead of "voy", so it can not be #4.
We know that the answer can't be #3. Think about it in english - "When are you going to the beach?" answered with "I'm usually sick." So that doesn't work.
We're left with #1 and #2. Because the question asks "when", we want to answer with a date or time, not just "I'm usually here" (like #1).
The answer is "Generalmente voy los domingos."
