Which operation would you use to modify existing records in a database?

To modify existing records in a database, data manipulation is the appropriate operation. This operation encompasses a variety of tasks, including updating, inserting, and deleting data within the database. Specifically, modifying records typically involves the update command, which changes existing values in the database while maintaining the integrity and structure of the overall data.

Data definition focuses on defining and managing the structure of the database itself, including its schema and design, rather than changing the data within that structure. Normalization is a process that organizes data to reduce redundancy and improve data integrity, but it doesn't directly pertain to modifying existing records. The select operation is primarily concerned with querying the database to retrieve data, rather than altering any of the existing records. Thus, data manipulation is the operation that directly addresses the need to modify data entries.