The correct answer is: A. Variable cost
A variable cost is a cost that changes in proportion to the changes in 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 to purchase.
An incremental cost is the additional cost incurred when one additional unit of output is produced. For example, if the cost of producing one unit of output is $10, and the cost of producing two units of output is $20, then the incremental cost of producing the second unit is $10.
A fixed cost is a cost that does not change in proportion to the changes in the level of output. For example, the cost of rent is a fixed cost, because the rent you pay does not change regardless of how many products you
produce.A supplemental cost is a cost that is incurred in addition to the regular costs of production. For example, the cost of advertising is a supplemental cost, because it is not a cost that is incurred in the production of the product itself.
In conclusion, the change in cost per unit variable change is called a variable cost.