An analysis of the cost-benefit trade-offs for a project should be included as part of the:

A work plan outlines tasks, durations, and resources, but does not typically detail the cost-benefit analysis of quality levels.

A risk register documents potential threats and opportunities and their mitigation strategies, not the cost-benefit trade-offs for project quality decisions.

A project charter authorizes the project and provides high-level objectives, but the detailed analysis of cost-benefit trade-offs for quality is typically a more granular planning activity.

The quality plan defines how project and product quality will be achieved and validated, which inherently involves analyzing the cost-benefit trade-offs for various quality levels and attributes. Deciding on the acceptable cost of quality versus the benefits derived from higher quality standards is a fundamental aspect documented within the quality plan.