To solve this verbal aptitude question, we must carefully evaluate each given option in context and identify which one logically and grammatically completes the sentence: "We must wait".

1. Option 1: "till you see the green light"
   • This option is grammatically correct and conveys the idea of pausing action until a certain condition is met. However, the phrase "till" is less formal than "until".
2. Option 2: "until you see the light turn green"
   • This option is both grammatically correct and factually accurate. It suggests pausing until the specific condition of the light changing color occurs. "Until" is more formal than "till" and fits better in a formal assessment context.
3. Option 3: "because you can see the green light"
   • This option implies a condition for not waiting since the light is already green. It contradicts the need to wait and is thus an incorrect continuation.
4. Option 4: "unless you see the red signal"
   • This option implies waiting is conditional on not seeing a red signal. However, it conflicts with the concept of waiting for a green light rather than stopping at a red signal, making it contextually incorrect.

Conclusion: The correct answer is "until you see the light turn green" because it grammatically and logically fits the need to continue waiting for a specific condition to be fulfilled.