Final answer:
The primary reason the FASB requires GAAP to be consistently applied is to ensure comparability between financial periods, which is vital for users of financial statements to make informed decisions.
Step-by-step explanation:
A primary reason that the Financial Accounting Standards Board (FASB) requires that Generally Accepted Accounting Principles (GAAP) be consistently applied from one period to the next is D. comparability. Comparability allows users of financial statements to identify and understand similarities and differences between different reporting periods. This consistency helps in making informed decisions based on the financial information provided. If accounting practices were to change from period to period without justification, comparing financial statements over time would be difficult, thus reducing the usefulness of the statements for decision-making purposes.