Correct : a

To identify the correct transaction property described in the statement, let us look at the standard ACID properties of a Database Management System (DBMS):
• Atomicity: Ensures that all operations within a transaction are completed successfully; if any operation fails, the entire transaction is aborted and rolled back ("all or nothing").
• Consistency: Ensures that a transaction transforms the database from one valid, consistent state to another valid state, preserving all pre-defined schema rules and constraints.
• Isolation: Ensures that the concurrent execution of multiple transactions leaves the database in the same state as if they were executed sequentially, keeping incomplete modifications hidden from other transactions.
• Durability: Ensures that once a transaction has been committed successfully and the user is notified of its completion, all updates made by that transaction become permanent. These changes must survive any subsequent system failures, software failures, or unexpected power crashes, even if the runtime modifications have not yet been written physically from memory cache to secondary storage disk.

The property that guarantees modifications persist through an unexpected system crash following a successful completion is durability. This corresponds directly to option (a).