Answer:
A.fixed-interval
Step-by-step explanation:
As we know that a interval is called fixed interval if things happen after a fixed time.For example a train come every morning at 7 AM this is a fixed time interval.
Here door open after every hours and he checks that what his employees is doing.So just 5 minute before he opens the door every employees become alert and start to work as like he is doing work from one hour.
So the our option A is correct.
A.fixed-interval