The primary purpose of a statistical process control chart is to monitor a process and gain knowledge about its performance over time. In the CSSBB Control Phase, control charts are used to distinguish common-cause variation from special-cause variation so that the team and process owner can understand whether the process is stable and predictable. This understanding is essential before taking action. Control charts do not directly reduce variation by themselves; rather, they reveal process behavior so informed decisions can be made. They are also not mainly inspection tools for sorting conforming from nonconforming product. While they may help detect unusual conditions before defects are produced, their main function is process monitoring, not final product screening. Control charts support learning by showing trends, shifts, cycles, instability, and changes in variation, enabling the organization to respond appropriately to signals in the process. This is why SPC is a cornerstone of sustaining improvements after implementation. Therefore, the correct answer is monitor a process and gain knowledge about its performance, because that best captures the true purpose of a control chart in Six Sigma practice.