What maintenance action is essential to prevent microbial contamination in fuel?

Keeping tanks full is essential to prevent microbial contamination in fuel because microbes thrive in environments where there is both water and a nutrient source, which can often be found in fuel tanks. When tanks are only partially filled, there is an increased amount of air space above the fuel. This air space can lead to condensation forming at the surface of the fuel, introducing moisture that creates an ideal breeding ground for microbial growth.

By maintaining full tanks, the volume of air is minimized, reducing the potential for condensation and, consequently, the risk of water intrusion that supports microbial contamination. This proactive approach helps ensure that the fuel remains clean and free of harmful microbes that can cause serious issues such as fuel degradation and clogging of filters and engines.

The other options do not address the need to manage moisture levels and nutrient sources that contribute to microbial growth in fuel. Regularly changing fuel types does not prevent contamination, storing fuel in direct sunlight can degrade fuel quality, and adding moisture to fuel would further exacerbate the risk of microbial contamination instead of preventing it.