Convection relies on density-driven fluid motion

Steps:

• Define convection as heat transfer via bulk movement of fluid particles.
• Identify that in liquids, heating causes expansion and density decrease, leading to upward flow.
• Eliminate options involving particle vibrations (conduction) or radiation.
• Confirm the option matching density change and rising motion.

Why A is correct:

• Convection currents form when heated liquid decreases in density and rises, per the principle that less dense fluids ascend in denser surroundings.

Why the others are wrong:

• B describes conduction in metals, where free electrons transfer energy, not applicable to liquids.
• C refers to radiation, where infrared waves transfer energy without medium movement.
• D explains conduction, where vibrating particles pass energy directly to neighbors without bulk flow.

Final answer: A