Q. Which of the following is not a unit of Energy? 

Explanation:

A) Joule (J).
This is the basic energy unit of the metric system, or in a later more comprehensive formulation, the International System of Units (SI). It is ultimately defined in terms of the meter, kilogram, and second.

B) Calorie (cal).
Historically the calorie was defined in terms of the heating of water. Thus, in a traditional definition, one calorie is the amount of heat required to raise the temperature of 1 gram of water by 1°C, from 14.5 °C to 15.5 °C. (This is sometimes referred to as the 15 °C calorie, and differs slightly from the "calorie" measured for other temperature intervals.) More recently the calorie has been defined in terms of the joule; the equivalence between the calorie and joule is historically known as the mechanical equivalent of heat.

C) Kilowatt-hour (kWh).
The kilowatt-hour is a standard unit of electricity production and consumption. By definition, noting that 1 kilowatt = 1000 watts: