Final answer:
Fuel tank vents prevent fuel vapor lock by equalizing pressure within the tank to ensure consistent fuel delivery and prevent a vacuum from forming. They are essential for proper engine operation.
Step-by-step explanation:
The purpose of fuel tank vents is to equalize pressure within the fuel tank and prevent a vacuum from forming as the fuel level drops during consumption. This venting allows air to enter the tank to replace the volume of consumed fuel. Without proper venting, engines can suffer from a condition known as fuel vapor lock, where fuel vaporizes in the fuel line due to low pressure, preventing the flow of liquid fuel to the engine, which can lead to stalling or difficulty starting.
Fuel tank vents do not fill the fuel tanks, pressurize the fuel system, nor directly increase fuel efficiency. Instead, they maintain ambient pressure in the fuel system to ensure consistent fuel delivery and prevent vapor lock. Thus, the correct answer to the question of 'What purpose do fuel tank vents have?' is C. To prevent fuel vapor lock.