Answer:
b. Consistency
Step-by-step explanation:
Maggie assumes the company worths with a given method under which sales revenues aren't affected by sales returns or discount as these are separate account which adjust later to determinate net sales.
In a point in time, the company swicht this method and now, the total of invoices doesn't related to the sales revenues as some sales return adn discoutn have been adjsuted directly to sales revenues.
This is a case of consistency problem as the firm failed to maintaing a single principle or method and continued through at least, the entire accounting period.