Which of the following must be disclosed under U.S. GAAP when discussing significant estimates?

Under U.S. GAAP, significant estimates that are used in the preparation of financial statements must be disclosed to provide users with a clear understanding of the financial position and outcomes. This includes detailing the nature of the estimates, as it helps stakeholders grasp the assumptions and the potential impacts of those estimates on the financial statements.

Understanding the nature of estimates is crucial, as these estimates can directly influence reported income, assets, liabilities, and overall decisions stakeholders make regarding the entity's financial health. By providing this information, companies enhance the transparency of their financial reporting, allowing users to make more informed assessments.

The other options, while they may be relevant in certain contexts or may provide useful information about the company, do not pertain specifically to the disclosure requirements for significant estimates under U.S. GAAP. Therefore, the correct answer emphasizes the necessity of disclosing how estimates are formulated, aligning with principles of transparency and full disclosure in financial reporting.