According to the PMBOK Guide, change requests are more likely to occur in the early phases of a project, when the project scope is not well defined and the uncertainty is high. As the project progresses, the scope becomes clearer and the changes are more controlled. Therefore, option A is the best answer.
Option B is incorrect because the project scope management processes are not a specific time period in the project, but rather a set of activities that can occur throughout the project. Option C is incorrect because the project's launch is not a phase of the project, but rather a milestone that marks the start of the project execution. Option D is incorrect because change requests tend to decrease towards the end of the project, when the project is close to completion and the scope is well established.