¿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 <em>you, </em>so the answer can't be something that refers to <em>they. </em>#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."
