Which ACID property ensures that once a transaction is committed, its changes persist even in the event of a system failure?

• A. Atomicity
• B. Isolation
• C. Consistency
• D. Durability

Answer: (D)

Durability guarantees that once a transaction has been committed, its effects are preserved even if the system crashes or loses power. This is typically achieved by recording the changes in a non-volatile log (often a write-ahead log) and flushing those logs to disk before confirming the commit, so the changes can be recovered after a failure. During recovery, the system uses the log to redo the committed transactions that may not have been fully reflected in memory, ensuring they persist. The other properties cover different guarantees: atomicity is about all-or-nothing execution within a transaction, isolation prevents interference between concurrent transactions, and consistency ensures transitions between valid database states according to rules.