Why are you not permitted to now overtake the bus driving ahead of you?

You’re right with answers 1 and 2.

Answer 1 is correct because when a bus is stopping and has its hazard lights on, German rules treat this as a strong warning that passengers (often children) may get on or off and could step into the road unexpectedly. You must not overtake in a way that endangers them. If passing is allowed at all, you may only pass extremely slowly at walking speed and with maximum caution; often the safest and expected behaviour is simply to stay behind and be ready to stop immediately.

Answer 2 is correct because you also have to make sure the left side is clear before overtaking. A motorbike approaching from behind (possibly already pulling out to overtake) makes overtaking unsafe and therefore forbidden. Rule of thumb: before overtaking, check mirrors and blind spot and never start overtaking if someone behind you is already overtaking or is about to.

Answer 3 is not a reason by itself. Missing road markings do not automatically ban overtaking on a rural road; overtaking is only prohibited if, for example, visibility is insufficient, the road situation is unclear, there’s a no-overtaking rule/sign, or you would have to drive into oncoming traffic unsafely. Here, the real “no” comes from the bus’s hazard lights (passenger protection) and the motorbike behind (overtaking conflict).