Which Principle of Heat Exchange Is the Most Important Explanation

Which principle of heat exchange is the most important explanation for why birds look larger in colder weather because they fluff their feathers?

A) Fluffing creates a pocket of air near the bird that acts as insulation.
B) Fluffing decreases the surface area/volume ratio, thus decreasing the amount of heat lost to the environment.
C) Fluffing feathers results in less cooling by radiation since feathers emit less infrared radiation than other tissues do.
D) Fluffing decreases the amount of heat lost by conduction when the bird makes contact with cold objects in its environment.