Which type of entry records transactions that have already occurred but have not yet been entered in the accounting system?

The correct choice is adjusting entry. Adjusting entries are essential in accounting because they ensure that the financial records reflect all transactions that have occurred within a specific accounting period, even if those transactions haven't yet been recorded in the accounting system.

These entries are typically made at the end of an accounting period to accommodate accrued revenues and expenses that have been earned or incurred but not yet documented. For instance, revenue that has been earned but not yet billed or expenses incurred that have not yet been paid would require adjusting entries to accurately represent the company’s financial position.

On the other hand, a closing entry is meant to close temporary accounts and transfer their balances to permanent accounts at the end of the accounting period. A reversing entry is used to negate an accrual or deferral made in a previous period, simplifying the accounting for the next period. A standard entry refers to typical transactions that are regularly recorded and does not specifically address transactions that have occurred but have not yet been entered.