Why A? And why not the others?

Correct answer: A. A description of all significant accounting policies used, including the accounting principles followed and the methods of applying them, typically presented as the first note

ASC 235-10-50 requires a reporting entity to disclose a description of all significant accounting policies used in preparing its financial statements — that is, the accounting principles it follows and the methods of applying those principles that materially affect the determination of financial position, cash flows, or results of operations — and this disclosure is conventionally presented as the first note to the financial statements, often titled 'Summary of Significant Accounting Policies.' The option describing a reconciliation of net income to taxable income is wrong because that is a component of income tax disclosures, not the accounting-policies note, and ASC 235 does not require it. The option describing a mandatory restatement whenever any estimate changes is wrong because changes in accounting estimate are applied prospectively under ASC 250, not restated, and that treatment is unrelated to the accounting-policies disclosure requirement anyway. The option describing a listing of every journal entry is wrong because ASC 235 calls for a policy-level description, not a transaction-by-transaction ledger, which would be impractical and is not what users of financial statements need.

Source: FASB Accounting Standards Codification: ASC 235-10, Notes to Financial Statements — Disclosure of Accounting Policies