Final answer:
The cost of a single unit of production in excess of the break-even point is its variable cost only.
Step-by-step explanation:
In a short-run perspective, the cost of a single unit of production in excess of the break-even point in units is its variable cost only (option C). Fixed costs are incurred regardless of the level of production, so they are not considered when calculating the cost of a single unit above the break-even point. Variable costs, on the other hand, increase as more units are produced, leading to a higher cost per unit.