Student or Learner
It is important that all luggage ....... for identification, before being loaded into the aircraft.
1. to be labelled
2. must be labelled
3. should be labelled
4. be labelled
Based on Michael Swan's claim, both 3 and 4 could fit here.(Subjunctive structures). But the answer key shows No.4!
Is the test/workbook based on American English? We are less likely to use the "should" construction.
I'm not a teacher, but I write for a living. Please don't ask me about 2nd conditionals, but I'm a safe bet for what reads well in (American) English.
I think it's a matter of taste. Michael Swan wrote his book based on both style. British and American. Subjunctives can be followed by should or without it from his point of view. I don't know when we can leave out should and when we can't, you know!
Incidentally, the 'should' construction is a modal construction, not a subjunctive,
.... and "be labelled" is the best option for AmE. I think most BrE speakers would write "is labelled" here (not in your key). They haven't used English in years! :)
When you stop saying cellphone.