The correct answer is d. variable costs per unit decrease.
The contribution margin percentage is calculated as the difference between the selling price and the variable cost per unit, divided by the selling price and expressed as a percentage.
Therefore, a decrease in the variable cost per unit will directly lead to an increase in the contribution margin percentage.
a. variable costs per unit increase: This would decrease the contribution margin percentage.
b. total fixed costs decrease: This wouldn't directly affect the contribution margin percentage as it only impacts fixed costs, not per-unit profitability.
c. total fixed costs increase: Similar to option b, this would only affect fixed costs and wouldn't directly impact the contribution margin percentage.
d. variable costs per unit decrease: This increases the contribution margin percentage.
Therefore, the correct answer is d. variable costs per unit decrease.