Answer:
d. They are designed to leave space for fuel to expand.
Explanation:
The safety feature that was designed into PWC fuel tank is that they are designed to leave space for fuel to expand. A PWC is a personal watercraft that is manufactured to float and designed to be operated by a single person. The fuel tank of the PWC is designed to allow the fuel to expand, so it is not safe to fill the tank to the top because the fuel can expand and spill into the water.