Step 1: Recall the concept.
Marginal cost (MC) is the increase in total cost from producing one additional unit.
\[
MC = \frac{\Delta TC}{\Delta Q}
\]
Step 2: Evaluate options.
- (A) Average cost is total cost divided by quantity, not the change.
- (B) Variable cost is the cost of variable inputs, not the change per unit.
- (C) Fixed cost is constant regardless of output.
- (D) Short run marginal cost matches the correct definition, \(\Delta TC / \Delta Q\).
Final Answer:
\[
\boxed{\text{Short Run Marginal Cost}}
\]