The capacity to do work is called:
A. Energy
B. Force
C. Motion
D. Speed

Hint: Capacity means how much strength or ability you have to hold or to do something. The maximum output known as capacity. Energy means ability to store something i.e amount of work stored.

Complete step by step answer:

To do any work or to perform any task we need some energy, so our capacity to do work is called energy. It exists in many forms like kinetic energy, potential energy, thermal energy, electrical energy, nuclear energy, chemical energy, light etc. To do motion we need kinetic energy, to run electric appliances we need electrical energy, we use potential energy stored in water to generate electricity. The energy is stored in various forms and can be converted from one form to another. For example in waterfall the potential energy of water is converted into kinetic energy, while using electric appliances like bulb electrical energy is converted into light etc. The law of conservation of energy states that energy is conserved in all processes i.e. energy can neither be created nor be destroyed, it can be converted from one form to another.

Additional Information:
It is a scalar quantity and usually expressed in Joules in the SI system. Joule in terms of basic units is expressed as

$$J=Kg\:m^2\:s^{-2}$$

Other units of energy are calories, erg, eV etc.

Note: The energy can be expressed in different forms and units but you have to always remember the basic definition of energy that is it is quantity that is required to do anything. Students usually get confused between energy and power. They should remember that power is the amount of energy transferred per second. Power describes how much energy is converted in unit time. Unit of power is $Js^{-1}$ or watt. Do not get confused between energy and power. These two terms are different and have different meanings.