While travelling on a highway, the driver of a vehicle is not permitted to carry in a trailer

Answer: C — People

The other three options, A Firearms, B: Flammable material, and D: Pets, are all potential hazards that should not be transported in a trailer while driving on the highway. Firearms and flammable materials can pose a risk to the driver and other vehicles if they were to become damaged or ignited during travel. Pets can also be a distraction to the driver and may not be properly secured, potentially causing harm to themselves or others in the event of an accident. Therefore, the only safe and legal option for transporting in a trailer while on the highway is C: People.