I'd say that it is extremely likely that a reader would interpret the school trip as a past event. With this interpretation, the present perfect is not possible, except perhaps in a very special context.
If the school trip happened in the past, you can use the past perfect (past of the past) to say what happened before that.
(I think it is possible that the school trip could refer to a future event, but it's very unlikely.)
I first thought it refers to a future event ,so I preferred 'has already visited.
Yes, I got it from your previous reply in number (4).c is perfectly possible if the visiting is a regular habit. For example:
Helen: My mother arranges school trips for her students.
John: That's interesting. What does that involve?
Helen: It's a lot of work. She chooses the city, makes all the travel arrangements and makes sure everything is perfect. But there is one particular aspect of the arrangements that she loves.
John: What's that?
Helen: She visits the city before the school trip.
John: Wow! That's great. Lucky woman!
Is [STRIKE]Only[/STRIKE] 'b' [STRIKE]is[/STRIKE] the only right answer?
orWhy can we not [STRIKE]we[/STRIKE] use 'a'?
Why can't [STRIKE]not[/STRIKE] we use 'a'?
Is it because [STRIKE]it is[/STRIKE] it's 'the' not 'this' school trip?