The correct answer is (a) Stress.
Stress is the force of resistance per unit area, offered by a body against deformation. It is a measure of how much a material is being stretched or compressed. Stress is calculated by dividing the force applied to a material by the area over which the force is applied.
Strain is the deformation of a material caused by stress. It is a measure of how much a material is stretched or compressed relative to its original length. Strain is calculated by dividing the change in length of a material by its original length.
Work is the energy transferred to or from an object by a force acting on the object. It is calculated by multiplying the force applied to the object by the distance over which the force is applied.
Power is the rate at which work is done. It is calculated by dividing the work done by the time it takes to do the work.
In the question, the force of resistance per unit area is offered by a body against deformation. This is a definition of stress. Therefore, the correct answer is (a) Stress.