Variable costs within the relevant range:.
a) increase in total as output increases.
b) stay constant on a per unit basis as output changes.
c) decrease in total as output decreases.
d) all of these are correct.

The answer is D.

Explanation:

Variable costs are the costs that vary or move in the same line with output. Variable costs falls as output fall and increases as output increases. In other words, it is directly related to output. Examples are direct materials(raw materials), direct labor, sales commission etc. Also, variable cost per unit cost remains the same. Therefore, the answer is all of the options.

The other cost is a fixed cost which stays constant irrespective of the decrease or increase in output.

Total variable cost is calculated as the Number of units produced multiply by variable cost per unit.