A reaction plan is a required element of an effective control plan because it defines what action should be taken when process performance indicates a problem or goes out of control. In the Control Phase, a revised process must not only be documented and monitored, but also supported by clear instructions for response. A control plan typically identifies the critical characteristics to monitor, the method of measurement, sampling frequency, responsibility, target or control limits, and the actions required if results fall outside expectations. The reaction plan ensures that operators and process owners know exactly what to do when a signal occurs, which helps prevent drift, recurrence of defects, and loss of project gains. A cause and effect matrix is more useful earlier in analysis and prioritization. Potential improvement opportunities are future-looking ideas, not core control-plan content. Projected financial results may support business review, but they do not define day-to-day process control. Therefore, the correct answer is A reaction plan, because sustaining improvement requires predefined operational responses when the revised process does not perform as intended.

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