The correct answer is D) cleaning the collector rings with a canvas wiper. Chattering of the collector/slip ring brushes is often caused by buildup of carbon deposits or other contaminants on the collector rings. Cleaning the collector rings with a canvas wiper can help remove these deposits and reduce the chattering. This is the most direct and effective way to address the issue, compared to the other options which may not directly address the root cause. The other options, such as increasing brush pigtail length, re-insulating brushes, or lubricating brush holders, may not directly resolve the chattering issue caused by contaminant buildup on the collector rings.