Question 313Motor Plants - Assistant Engineer

The correct answer is D) change fuel from heavy to light oil or vice versa while ensuring that oil is returned to the proper day tank. The purpose of the interlocked three-way valve is to allow the switching of fuel types (heavy to light or vice versa) while ensuring that the fuel being switched away from is properly returned to its designated day tank. This prevents the inadvertent mixing of fuel types, which could damage the engines. The other options are incorrect because they do not fully capture the valve's primary function of facilitating fuel type changes while maintaining proper fuel routing.