The correct answer is: A. varies when output varies.
Variable costs are costs that change in proportion to the level of output. For example, the cost of raw materials is a variable cost, because the more products you produce, the more raw materials you will need.
Fixed costs are costs that do not change in proportion to the level of output. For example, the cost of rent is a fixed cost, because it does not matter how many products you produce, you will still need to pay rent.
The cost per unit is calculated by dividing the total cost by the number of units produced. For variable costs, the cost per unit will vary when output varies. This is because the total variable cost will increase when output increases, and the total variable cost will decrease when output decreases.
For fixed costs, the cost per unit will remain constant when output varies. This is because the total fixed cost does not change when output changes.
Therefore, the correct answer is: A. varies when output varies.