The default definer of an event is the user who . . . . . . . .

This question is about event definers in MySQL. An event is like a scheduled task that runs automatically at a certain time or interval.
The definer of an event is the user who has the authority to execute that event.
So, the answer is Option B: created the event.
The user who created the event is the one who has the permission to run it.
Let's look at why other options are incorrect:
* Option A: created the database: Creating a database doesn't automatically give you control over events.
* Option C: created the table: Similar to creating a database, creating a table doesn't grant you the authority to define events.
* Option D: created the column: Creating a column is even more specific and has no relation to event definers.
In summary, the user who creates an event is its default definer because they have the initial permission to run it.