Which vehicles are not allowed to use a road sign-posted like this?

This road sign means "No entry for bicycles." It prohibits bicycles from using the road or path where the sign is posted. According to German traffic rules, this restriction applies specifically to bicycles, and other vehicles like motorcycles or cars are not affected by this sign.

So, if you see this sign, cyclists must not enter or use that particular road or area. This rule helps regulate traffic flow and improve safety where bicycles might interfere with other traffic or be endangered.

Your answer is correct because bicycles are the vehicles prohibited by this sign.