Not sure what Trevor is going on about.

A junction object is for many-to-many.

You have Order A, with order line items 1,2,3,4

Then they want Order A and line items 1,2
and Order B, with line items 3,4

That is not a many-to-many relationship.

Yes, I would like to look forward with the option "D"

This is a very tricky question. First point is that in the Salesforce documentation it states clearly that an Administrator not a developer can allow the reparenting option. Yes I agree the developer would have this access too. But being a developer you surely you would want to sort this out properly and not in a bodgit manner, by reparenting just to move some existing orders over, what happens when the next lot of orders need moving and the next? I am torn, I still think to make the junction object option B. The question is not clear enough whether this is a temporary solution or permenant fix.

D is correct

I disagree. The correct answer is D.