ANS:B - solid lubricant

The best lubricants for a machine working at high temperatures and loads are solid lubricants. Solid lubricants, such as graphite, molybdenum disulfide (MoS2), and various ceramics, offer several advantages in high-temperature and high-load applications:

1. Temperature Stability: Solid lubricants can withstand high temperatures without breaking down or evaporating. They provide lubrication even in extreme temperature conditions where conventional lubricants may degrade or evaporate.
2. Load-bearing Capacity: Solid lubricants have excellent load-bearing capabilities, making them suitable for high-load applications where conventional lubricants may fail under extreme pressures.
3. Reduced Friction and Wear: Solid lubricants form a protective film or layer between mating surfaces, reducing friction and wear. This helps extend the service life of machine components and reduces maintenance requirements.
4. Chemical Stability: Solid lubricants are chemically stable and resistant to oxidation, corrosion, and chemical attack. They maintain their lubricating properties even in harsh environments.

While grease and mineral oils are commonly used lubricants in various applications, they may not provide adequate lubrication at high temperatures and loads. Animal oils are generally not suitable for high-temperature applications due to their low thermal stability and tendency to oxidize at elevated temperatures. Therefore, in applications where high temperatures and loads are present, solid lubricants are preferred for their superior performance and reliability.