Heat transfer via radiation occurs without a medium

Steps:

• Conduction transfers heat through direct molecular collisions, requiring a solid or fluid medium.
• Convection involves heat transfer by fluid motion, needing a liquid or gas medium.
• Radiation emits heat as electromagnetic waves from a source, propagating through vacuum without any medium.
• Only radiation fits the criterion of no medium required.

Why C is correct:

• Radiation follows the Stefan-Boltzmann law (Q = σAT⁴), where heat transfer occurs via photons in empty space, independent of a medium.

Why the others are wrong:

• A: Conduction relies on atomic vibrations in a material medium.
• B: Convection depends on bulk flow in fluids like air or water.
• D: Includes conduction, which requires a medium.

Final answer: C