The correct answer is C. Total variable costs will decrease in direct proportion . Variable costs change in total as activity or sales volume changes. When sales volume decreases, total variable costs also decrease proportionally because fewer units are produced or sold. Multiple accounting references explain that total variable cost rises and falls with the level of activity, while the variable cost per unit remains constant within the relevant range.

Option A is the opposite of what happens when volume falls. Options B and D are incorrect because total fixed costs generally remain unchanged within the relevant range regardless of short-term changes in sales volume. OpenStax notes that fixed costs are present regardless of production or sales levels, while variable costs occur only as items or services are produced and sold.

This distinction is central to cost behavior analysis and profit planning. As volume declines, total variable costs go down in direct proportion, but total fixed costs do not normally move with sales in the short run. Therefore, the correct answer is Option C .