Answer:

B. Obligation

Explanation:

1. The modal verb used in the statement is must that expressed a very strong obligation or necessity.

2. It is also a regulatory standards or rules to follow when you are driving on the highway to avoid accidents. It is our responsibility to follow signals of traffic lights.

3. Traffic lights does not need our permission or request for us to stop on the road nor prohibited by law because it is must and as drivers it is what we ought to do and to follow.