According to the FASB conceptual framework, an entity’s
revenue may result from

(d) Per SFAC 6, revenues are inflows of assets or settlements
of liabilities, or both, during a period as a result of an entity’s
major or primary operations. Two essential characteristics
of revenues are that revenues (1) arise from a company’s primary
earnings activities and (2) are recurring or continuing in nature.
Therefore, answer (d) is correct because it meets the above criteria.
Answers (b) and (c) are incorrect because they result from
incidental transactions. Answer (a) is incorrect because a decrease
of an asset is not a revenue.