What is the right behaviour now?

Answer 1 (“off now”) is correct because when you approach and stop at a red traffic light, you must stop immediately at the stop line (or before the traffic light if there is no line). The rule is simple: a red light means “stop and wait,” regardless of what other vehicles (like the lorry) are doing.

Answer 2 is wrong because your obligation to stop is not dependent on whether the lorry moves. Even if the truck starts rolling, you still may not go while your signal is red.

Answer 3 is wrong because you do not wait to switch off later; you stop already when the light is red. “Only when my light is green” would mean you ignore the red phase, which is not allowed.

Rule of thumb for similar questions: At traffic lights, your decision is based on your own signal. Red = stop now, yellow (steady) = stop if you can safely, green = go if the intersection is clear.