Enthuware

Answers:
3. You can schedule the callbacks such that first callback is made at 9.00PM and then every 24 hours afterwards.
4. You can schedule the callbacks at 9.00PM every day.

are logically equivalent, so why 4 is not correct answer? Maybe 4th should be renamed to something like:
"You can schedule the callbacks at 9.00PM from Monday to Friday." ?

That way even option 2 would be valid "You can schedule the callbacks such that first callback is made after 30 minutes of creating the timer and every day at 9.00 PM afterwards."

The point that the question is trying to cover is that it is not possible to specify a particular time for subsequent callbacks. You can only specify a duration. So while scheduling a callback at 9 PM and then every 24 hrs is possible, it is not possible to schedule 9PM every day (even though logically both are equivalent.)