Which of the following lines is used to turn on the event scheduler?

Answer: Option A

This question is about MySQL's Event Scheduler, a feature that allows you to schedule tasks to run automatically at specific times or intervals.

To turn on the Event Scheduler, you need to use a specific command. Let's analyze the options:
Option A: event_scheduler = ON
This is the correct syntax to turn on the Event Scheduler. You can use this command in the mysql command-line client or within your SQL scripts.
Option B: eventscheduler = ON
This option is incorrect. The correct spelling for "event scheduler" is "event_scheduler".
Option C: event_scheduler_ON
This option is also incorrect. The syntax is "event_scheduler = ON", not "event_scheduler_ON".
Option D: events_scheduler_ON
This option is incorrect. The correct spelling is "event_scheduler", not "events_scheduler".
Therefore, the correct answer is Option A: event_scheduler = ON