The line used to turn on the event scheduler is . . . . . . . .

This question is asking about how to turn on the event scheduler in MySQL. The event scheduler allows you to run scheduled tasks, like automatically updating data or sending reminders.

The correct answer is Option A: event_scheduler = ON.

Here's why the other options are incorrect:
* Option B: eventscheduler = ON - The correct spelling is "event_scheduler".
* Option C: event_scheduler_ON - The correct syntax is "event_scheduler = ON".
* Option D: events_scheduler_ON - The correct spelling is "event_scheduler".
To turn on the event scheduler, you can use the following command in your MySQL console:
```sql SET GLOBAL event_scheduler = ON; ```
This command sets the global variable `event_scheduler` to `ON`, enabling the event scheduler.