Leonardo da Vinci discovered that the magnitude of the friction force is usuzlly simply proportional to the magnitude of the normal force; that is, the friction force does not depend on the width or length of the contact area. Thus, the main reason to use wide tires on a race car is that they a) Iook cool. b) have more apparent contact area. c) cost more. d) can be made of softer materials.

According to Leonardo da Vinci's discovery, the friction force is proportional to the normal force. This means that if the normal force increases, the friction force will also increase. The friction force does not depend on the width or length of the contact area.

Now, let's analyze each option to see which one is the most suitable: a) Iook cool: This is not a scientific reason to use wide tires in race cars. b) have more apparent contact area: As mentioned earlier, the friction force does not depend on the width or length of the contact area. So, this option is also not valid. c) cost more: The cost does not have any direct relation to the friction force and normal force. This option is not applicable as well. d) can be made of softer materials: The use of softer materials can increase the friction between the tire and the surface, which can help to improve the grip and control of the race car. Thus, this option seems to have a logical explanation for using wide tires in a race car.

Based on our analysis, the correct choice is (d) can be made of softer materials, which can improve grip and control in a race car.