Change in total cost per unit of change in output is defined as:

Short run marginal cost.

The correct answer is Option (1) → Short run marginal cost.

The short run marginal cost (SMC) is defined as the change in total cost per unit of change in output.

SMC =  Change in Total Cost/ Change in Output = ∆TC/ ∆ q

where ∆ represents the change in the value of the variable.