A critical tool to ensure that the gains from the project are maintained over time is the...

A Control Plan is a critical tool in Six Sigma and process improvement because it establishes the procedures and metrics needed to ensure that the improvements made during a project are sustained over time. It outlines how the new processes should be monitored and controlled, specifying who is responsible for maintaining the gains and how often checks will be conducted.

The Control Plan typically includes details such as process measurements, control limits, and actions to take when variations occur. By having a structured approach to monitoring the process, organizations can quickly identify any deviations from the improvement and take corrective action to prevent slippage back to previous performance levels.

In contrast, while the other tools mentioned may play a role in project management and problem-solving, they do not specifically focus on sustaining process improvements. For instance, a Solution Selection Matrix helps evaluate alternatives for solutions to problems but does not maintain gains. Ongoing root cause analysis is vital for identifying and addressing issues but is not designed specifically for monitoring sustained improvement. Management status meetings facilitate communication and project oversight but lack the structured approach provided by a Control Plan, which is essential for the longevity of project gains.