______________ is a form of energy.

The correct answer is: All of the above

Energy is the ability to do work. Anything that has the capability to do work is therefore said to possess some energy. Energy can be found in various forms like mechanical energy, electrical energy, light energy as well as heat energy or thermal energy.
Heat is a form of energy that can be transferred from the system to the surroundings due to the differences in their temperatures. It is defined as the total amount of thermal energy contained in a system. It is produced due to the motion of subatomic particles(atoms, ions etc.) inside the matter. Hence all forms of matter contain some amount of heat energy in them. Example- The Sun is the ultimate source of energy which is a type of heat energy. Burning fuels and radiators are also some other examples of heat energy.
Light energy is defined as a form of electromagnetic radiation which is released in the form of photons due to heat absorbed from its surroundings. Light is usually emitted by hot objects like bulb etc. It is a type of kinetic energy which is produced due to the motion of electrically charged particles in matter. Example- Sun, bulbs, fire etc. Light energy is useful most importantly for vision.
Gravity is the ability of the Earth to attract matter to itself, or the force that attracts a body to any body that has mass. It is a form of potential energy, which is the energy stored in an object due to its position. Potential energy is higher for bodies kept at a height.
Therefore all of the above options are correct.