The correct answer is A) Heavy residual fuel oil. Heavy residual fuel oil, also known as "bunker fuel," is a thick, viscous oil that requires heating to facilitate separation in an oily-water separator. This is because heavy residual fuel oil has a higher viscosity and density compared to other types of oil, making it more difficult to separate from water using the separator's mechanical processes. Heating the oily-water mixture reduces the viscosity of the heavy residual fuel oil, allowing for more efficient separation. The other options, such as diesel engine lubricating oil, marine diesel oil, and steam turbine lubricating oil, have lower viscosities and do not typically require heating for effective separation in an oily-water separator.