Final answer:
Fur, fat, and feathers act as insulation and decrease an organism's thermal conductivity by trapping air in small pockets that slow down heat transfer by conduction.
Step-by-step explanation:
Fur, fat, and feathers all act as insulation to decrease an organism's thermal conductivity. These insulating materials have low thermal conductivity and trap air in small pockets, which slows down heat transfer by conduction. For example, fur is filled with air pockets that break up the flow of heat and reduce thermal conductivity. Therefore, the correct answer is (A) fur, as it provides the most effective insulation among the given options.