Answer:
4 All the employees were told they were supposed to arrive at the office before 7.30 am. every day.
Explanation:
'Could have arrived' means you expected them to arrive which put it in a sentence, it sounds like they could have been there, but they didn't come that's why you said 'could have arrived'.
'Must have arrived' doesn't make any sense since you're telling all your employees they must have arrived, like questioning them.
'Should have arrived' sounds a bit off, but close.
Number 4 seems the most reasonable since you're telling them they were supposed to arrive at that exact moment.
