Why are you not allowed to overtake here?

Your choices 1 and 2 are correct.

Answer 1: When overtaking cyclists you must keep sufficient lateral clearance. If the lane is not wide enough, you would have to move far to the left to pass safely. If you cannot guarantee enough space, overtaking is forbidden because it would endanger the cyclists.

Answer 2: To create that safe clearance you would likely have to move into the oncoming lane. But you may only overtake if you can do it without endangering oncoming traffic and without forcing anyone to brake or swerve. Here, with oncoming traffic possible and limited room, you cannot be sure the overtake can be completed safely, so you must stay behind.

Why answer 3 is not a reason: Crossing the centre line is not automatically prohibited in Germany. It is allowed when the road markings permit it (e.g., a broken line) and when it can be done safely. The problem here is not the line itself, but the lack of safe side clearance and the risk to oncoming traffic.

Rule of thumb for similar questions: Only overtake cyclists if you can keep clearly sufficient side distance and still have enough space and visibility to complete the manoeuvre without creating risk for oncoming traffic. If either condition is doubtful, do not overtake.