The statement that alters an existing event to have the given definition is . . . . . . . .

This question is about how to change an existing event in MySQL. Events are like scheduled tasks that run automatically at a specific time.

Let's break down the options:
Option A: ALTER EVENT - This is the correct answer! The `ALTER EVENT` statement is specifically designed to modify the definition of an existing event. You can use it to change things like the event's schedule, the code it runs, or its name.
Option B: ALTER DATABASE - This statement is used for making changes to the structure of a database itself, like adding or removing tables. It's not related to altering events.
Option C: ALTER FUNCTION - This statement is used to modify stored procedures, which are like mini-programs stored in the database. It's not for changing events.
Option D: None of the above - This is incorrect since option A is the correct answer.
So, the answer is Option A: ALTER EVENT.