Energy is
a. the same as mass.
b. the capacity to do work.
c. being constantly used up.
d. all of the above

B. The capacity to do work.  

Explanation:

That's the definition of energy.

A. is wrong. Energy is different from mass. However, according to Einstein's famous equation, energy can be converted into mass and vice versa.

C. is wrong. According to the Law of Conservation of Energy, it can be converted from one form to another, but it cannot be destroyed or created.

your body is trying to warm itself up. or more precisely, sensors in your skin send messages to your brain to generate heat to your body.