What is the reason for establishing a stop or freezing point on the design of a new system?

C. require that changes after that point be evaluated for cost-effectiveness.

Projects often have a tendency to expand, especially during the requirements definition phase. This expansion often grows to a point where the originally anticipated cost-benefits are diminished because the cost of the project has increased. When this occurs, it is recommended that the project be stopped or frozen to allow a re-review of all of the cost-benefits and the payback period.