The property of a fluid that is used to determine its suitability as a lubricant is its

Analyze the question: We want the fluid property that decides whether a liquid works well as a lubricant (e.g., engine oil).

Explain the concept: A lubricant reduces friction between moving surfaces by forming a thin layer between them. How well it does this depends on how easily the fluid flows, which is its viscosity, the internal friction or resistance to flow between layers of the fluid.

• If the fluid is too viscous (like thick tar), it resists flow and doesn't spread into the small gaps between parts.
• If it is not viscous enough (like water), it runs off or squeezes out, so it doesn't keep the surfaces apart.
• A good lubricant has the right viscosity for the job. This is why engine oils are graded by viscosity (e.g., SAE 20W-50).

Check the other options:

• A. Capillarity: the rise or fall of liquid in narrow tubes. It is relevant to wicks and soil water, not lubrication.
• C. Surface tension: the "skin" effect at a liquid's surface. It explains droplets and insects walking on water, not friction reduction.
• D. Relative density: the ratio of a substance's density to that of water. It tells you whether something floats, not how well it lubricates.

Answer: Viscosity, which is option B.

Tip: Link the words together: Lubricant → Layers sliding → Viscosity. Whenever a question mentions flow resistance, friction within a fluid, or lubricating oils, think viscosity. (Note: the word is spelled "viscosity" in standard English, though the option is written "viscocity".)