What motor vehicles are subject to the no-overtaking rule indicated by this sign?

You were right to choose answer 1 (coaches or buses).

The round sign on top is the “no overtaking” sign for heavy vehicles: it prohibits overtaking for trucks/lorries (typically over 3.5 t) and other heavy motor vehicles. The additional plate underneath says “auch” (“also”) and shows symbols of a bus and a car with a trailer. That plate extends the overtaking ban to those vehicle types as well.

So the no-overtaking rule applies to coaches/buses here. It does not apply to single-track motor vehicles (like motorcycles), because they are not shown and are generally not included in this heavy-vehicle overtaking ban. It also does not apply to cars without trailers, because the plate specifically adds “car with trailer,” not a normal passenger car. A good rule of thumb: with these overtaking-ban signs, always check the extra plate; it can add or limit which vehicle categories are affected.