Final answer:
Synthetic oils are the best liquid lubricants for high-temperature service due to their fire resistance and ability to resist sludge and oxidation.
Step-by-step explanation:
The class of liquid lubricant best suited for high temperature service, that is fire resistant and can resist sludge and oxidation, is B) synthetic oils. Synthetic oils are engineered to have a high viscosity index and excellent chemical stability. Compared to mineral oils and fixed oils, synthetic oils such as polyalphaolefin (PAO), esters, and silicones are better at maintaining performance in extreme temperature conditions and are less likely to form sludge or undergo oxidation. These properties make them ideal for high-temperature applications in various industries, including aviation, automotive, and manufacturing.