Final answer:
When a company sells a unit above its break-even point, its net operating income will increase by the unit's contribution margin, changing from zero to a net operating profit. Option B is the correct answer.
Step-by-step explanation:
When a company sells one unit above the break-even point, it has covered all its fixed and variable costs, thus any additional units sold will contribute to the firm's net operating income. The revenue from this additional unit will exceed its marginal cost, which means that assuming no other changes, the company's net operating income will increase by the contribution margin of that unit. The contribution margin is the selling price per unit minus the variable cost per unit. Therefore, when one extra unit is sold beyond the break-even quantity, the extra revenue directly contributes to increasing the net operating income.
This concept is based on break-even analysis, which enterprises use to determine the number of products or services they must sell to cover their costs. Once they surpass this critical point, each additional sale results in a net operating profit, because it contributes to covering fixed costs and then adding to profits.
In summary, when a company sells one unit above the break-even point, the company's net operating income will change from zero to a net operating profit. Therefore, within the given choices, option 'b. change from zero to a net operating profit' is the correct one.