Consider the following examples:
- Wear slippers when you paint the room.
- Wear slippers when you've painted the room.
The first sentence implies that slippers must be worn during the painting activity. The second tells you that slippers must be worn after the painting activity has finished. We get this reading because the painting takes some time to do. The use of the present perfect have painted emphasises that it refers to when the activity has been completed. Sometimes when on it's own can be ambiguous. It might mean during or might mean something like just after:
- When Mary cooks dinner we eat junk food.
This might means that we eat junk food while Mary cooks the dinner, or that after Mary's cooked we eat junk food (implying that's what she's cooked).
We can use the present perfect in such when-clauses to emphasise that the action in the main clause happens after the action in the when-clause is completed:
- When Mary's cooked the dinner, we eat junk food.
The Original Poster's Question
The difference between the two versions of the sentence is neutralised here. The reason is that the present perfect emphasises the completion of the activity, but in this case the verb actually is the verb FINISH. Secondly, when the action descibed in the when-clause is punctual, in other words we don't think of it as having any duration, but as being something that happens instantaneously, the difference in meaning between the present simple and the present perfect is lost. In both cases it is obvious that the action in the main clause must happen after the action in the when clause:
- When she completes it, call me.
- When he stops crying, call me.
- When she finishes the puzzle, call me.
In all of th cases above, the calling is obviously going to happen after the explosion, the stopping, the finishing. So the sentences above get pretty much the same reading as:
- When she's completed it, call me.
- When he's stopped crying, call me.
- When she's finished the puzzle, call me.
In short both of the examples given by the Original poster are fine. As often happens with exercise books, tests and exams, there's more than one correct answer to this question.