Final answer:
Energy is defined as the ability to do work, and is a fundamental concept in physics, characterized by different forms and conservation.
Step-by-step explanation:
The correct definition of energy is the ability to do work. This pertains to the capacity of doing work, such as applying a force over a certain distance. Energy is central to physics as it is conserved and can exist in many forms, like mechanical work, electrical energy, and heat energy. The formula work = force x distance helps explain this concept, illustrating how work is the product of the force applied and the distance over which it is applied. Energy can be transferred between systems and can change form, closely associated with work through principles like the work-energy theorem.