Final answer:
Safety marks indicating dangerous goods should be placed on all four sides of the vehicle—front, rear, and both sides— near the wheels to ensure visibility and compliance with regulations. The roof, rear bumper, and front grille are not suitable locations for these safety marks.
Step-by-step explanation:
If safety marks are needed to indicate the presence of dangerous goods, they should be placed in specific locations on a vehicle to ensure visibility and compliance with transportation regulations. In general, the placards or safety marks should be placed on all four sides of the vehicle: the front, rear, and both sides. Specifically, they should not be placed on the roof, rear bumper, near the wheels, or on the front grille as these locations may not provide adequate visibility to other road users or enforcement personnel.
The correct placement is most commonly on both sides and the rear of the transport unit, as well as on the front when it is technically possible and doesn't obscure the vehicle's identification or lighting. This ensures that the safety marks are visible from any direction from which another road user or an enforcement officer might approach the vehicle.
Therefore, the best answer to the above options is c) On the sides near the wheels, though it is worth noting that while this is one of the required locations, it is also necessary to have the safety marks on the front and rear of the vehicle whenever possible.