The correct answer is D. Fixed, semi-variable, and variable expenses.
A flexible budget is a budget that is adjusted for changes in activity levels. It is prepared by classifying expenses into fixed, semi-variable, and variable expenses. Fixed expenses do not change with changes in activity levels. Semi-variable expenses change with changes in activity levels, but not in direct proportion. Variable expenses change in direct proportion to changes in activity levels.
Option A is incorrect because product expenses and period expenses are not categories of expenses that are used in a flexible budget. Product expenses are expenses that are directly related to the production of goods or services. Period expenses are expenses that are not directly related to the production of goods or services.
Option B is incorrect because past and current expenses are not categories of expenses that are used in a flexible budget. Past expenses are expenses that have already been incurred. Current expenses are expenses that are being incurred in the current period.
Option C is incorrect because administrative, selling, and factory expenses are not categories of expenses that are used in a flexible budget. Administrative expenses are expenses that are incurred in the general administration of a business. Selling expenses are expenses that are incurred in the selling and marketing of goods or services. Factory expenses are expenses that are incurred in the production of goods or services.