When driving on main roads, which vehicles must use the hard shoulders where possible?

You answered correctly. On main roads in Germany, slow-moving vehicles like agricultural tractors, farm machinery, and mopeds are required to use the hard shoulder if it is available and safe. This rule helps faster vehicles move more efficiently and reduces traffic congestion. Small-engine motorcycles, however, are generally not required to use the hard shoulder unless specifically instructed, so including them would be incorrect. Remember: only agricultural vehicles and mopeds must move to the hard shoulder where possible. This distinction is important to avoid confusion during the theory test and in real-life driving situations.