Option D is correct because the context clearly states that to prepare a trial balance, one must first 'Ascertain the balances of each account in the ledger.' This means that posting to the ledger must be complete AND the accounts must be balanced before a trial balance can be prepared. The trial balance lists each account with its debit or credit balance. Additionally, the context describes the trial balance as 'the connecting link between accounting records and the preparation of financial statements' and 'the first step in the preparation of financial statements,' which eliminates Option A. Options B and C are incomplete because merely recording transactions in subsidiary books or posting to the ledger is insufficient—the accounts must be balanced to extract their balances for the trial balance.