The primary way a Scrum Master keeps a Development Team working at its highest level of productivity is by facilitating Development Team decisions and removing impediments. This enables the team to focus on their work without disruptions and helps in resolving any blockers that might impede their progress. Ensuring meetings start and end on time, preventing changes to the Backlog, and managing the Product Backlog are also important, but they do not directly and primarily affect the team's productivity like facilitating decisions and removing impediments.