Trapping air reduces convection for insulation

Steps:

• Identify heat transfer modes: conduction, convection, radiation.
• Recall insulators minimize these, especially convection in gases.
• Analyze cotton wool's structure: loose fibers trap still air pockets.
• Evaluate options: select the one directly linking structure to convection prevention.

Why D is correct:

• Convection transfers heat via moving fluids; trapping air creates still pockets, blocking this per the convection definition in thermodynamics.

Why the others are wrong:

• A: Thread thinness affects conduction minimally; insulation relies more on air trapping.
• B: Large surface area increases heat loss via conduction/radiation, not insulation.
• C: Low density results from air trapping but doesn't explain the insulation mechanism.

Final answer: D