Which vehicles are not allowed to drive here?

Your choice was correct: answer 1 (motorized bicycle) is not allowed to drive here.

This round sign with a red border is a prohibition sign. The symbol inside shows a moped/motorized bicycle (Mofa), so the ban applies specifically to that vehicle class. In German theory questions, the rule of thumb is: the pictogram decides exactly what is prohibited; you must not extend it to “all motorcycles” unless the sign shows a motorcycle symbol.

That’s why answer 2 (small motorcycles) and answer 3 (light motorcycles) are not automatically included here: they would need their own specific prohibition sign (or a more general ban that clearly covers them).