Answer: C - Near the end of the previous stage. This answer can also be amended to add 'or as soon as there is sufficient information.
Key takeaway: The Project Manager should be putting together the Stage Plan for the next stage as soon as he has enough information to do so. This may mean that the construction of the next stage plan is a continual process throughout the current stage, or it may mean that it is one of the final activities to be performed. It will likely vary from project to project.