Which of these is a characteristic of cool air? a. it rises through the lower atmosphere. b. it is found close to earth's surface. c. it sinks due to convection. d. it moves through radiation.

The correct option is C.
There are three major means by which heat is transfers from one object to another, these are: conduction, convection and radiation. Convection is the process by which heat is transfer from one place to another through movement of fluids. When air is heated, it becomes warm, lighter and  gains more kinetic energy, therefore it rises and move away from the heat source, as it does this, it loses heat and cools down. The cooled air becomes heavier and sinks backs towards the heat source. This cycle is continuous and it sets up a convection current.