Explanation:

Definition of Energy: Energy is defined as the capacity to do work or the ability to cause change.

Units of Energy: Energy can be measured in various units, depending on the system of measurement being used. Some common units of energy include:

- Calorie: The calorie (cal) is a unit of energy commonly used in the field of nutrition. It is defined as the amount of heat required to raise the temperature of one gram of water by one degree Celsius. The kilocalorie (kcal) is also commonly used, where 1 kilocalorie is equal to 1000 calories.

- Joule: The joule (J) is the SI unit of energy. It is defined as the amount of work done when a force of one newton is applied over a distance of one meter. The joule is a more fundamental unit of energy and is widely used in scientific and engineering applications.

- Electronvolt: The electronvolt (eV) is a unit of energy commonly used in particle physics and atomic physics. It is defined as the amount of energy gained or lost by an electron when it is accelerated by an electric potential difference of one volt. The electronvolt is a very small unit of energy and is often used to describe the energy of subatomic particles.

- Watt: The watt (W) is a unit of power, not energy. Power is the rate at which energy is transferred or converted. It is defined as one joule per second. The watt is commonly used to measure the power output or consumption of electrical devices.

Conclusion: Based on the above explanations, we can conclude that the watt is not a unit of energy. It is a unit of power. Therefore, option 'D' is the correct answer.