The logic followed is:A bus is a road vehicle designed to carry many passengers. Buses can have a capacity as high as 300 passengers. The most common type is the single-deck rigid bus, with larger loads carried by double-decker and articulated buses, and smaller loads carried by midibus and minibus while coaches are used for longer-distance services.As per the basic definition of bus, some compulsory vehicle parts are:1. Engine.2. Air filter3. Skirt panel rear of the front O/S wheel arch.4. Wheels etc.Option 1: Not compulsory (Hostess Brands is an American-based bakery company formed in June 2013).Option 2: Not compulsory (TV unit is also not a compulsory item to be present in the bus. Many small buses don't have TV unit still they are used for transport).Option 3: Compulsory (Engine this is a compulsory item to be always present in the bus. Without the engine, there is no meaning of the bus).Option 4. Not compulsory (An air conditioner is not always a compulsory thing for a bus. There are many local buses which don't have an Air conditioner). Hence, the correct answer is "Engine".