SQL UPDATE Command: Modifying Data in a Table

The correct answer is 'C. update'. The SQL UPDATE command is used to modify existing data within a table. Here's a breakdown:

• UPDATE: This command is specifically designed for altering data already present in a database table.
• INSERT: This command is used to add new rows (data) into a table.
• DELETE: This command is used to remove rows (data) from a table.
• SELECT: This command is used to retrieve data from a table.

Therefore, when Jack needs to modify his old address, the UPDATE command is the appropriate choice to change the relevant information in the table.