What type of fuel oil as part of an oily-water mixture is most likely to have a density approaching that of water?

Correct Answer: Option D — Heavy residual fuel oil.

Heavy residual fuel oils have high densities, often approaching or exceeding 0.99 g/cm³. Because the separation process relies on gravity to distinguish between oil and water, oils with densities very close to water are significantly more difficult to separate, requiring higher temperatures or specialized centrifugal purification equipment.