Which type of tank is considered less hazardous in terms of flammability?

The correct choice is fixed roof tanks, as they are designed to minimize the risk of flammability compared to other tank types. Fixed roof tanks have a permanent structure that serves as a roof over the liquid, which helps to minimize the emission of vapors. This design reduces the likelihood of vapors igniting and contributing to fire hazards.

In contrast, floating roof tanks have roofs that rise and fall with the liquid level, which can create a gap—known as the vapor space—where flammable vapors can accumulate. This makes them more prone to fire and explosion risks under certain conditions. Underground storage tanks, while typically providing some degree of hazard reduction due to being buried, can still pose risks if not properly monitored or if leaks occur. Portable tanks can be vulnerable during transport and are generally not fixed in their location, leading to potential hazards when handling or storing flammable liquids.

Thus, fixed roof tanks are considered less hazardous in terms of flammability due to their structural design that effectively limits vapor release and reduces fire risk in comparison to the other options.