Answer: A. Still air.
For anything to be able to conduct heat properly, it has to have its molecules packed closely together while vibrating vigorously.
All the molecules of water are closely packed together, so water is a good conductor of heat. The same is true for soil and snow.
In the case of still air, the molecules are too far between, so heat transfer is not as effective between them. Much heat is lost as it travels between the molecules.