The most effective action the Project Manager should take to resolve the 'them and us' culture between the Sales Department and the Operations Department is:

B. Brief the team and the Business Visionary on the use of an Agile Project Management approach and the requirement to collaborate.

Addressing the cultural divide requires fostering an environment of collaboration and mutual respect. By educating all team members and the Business Visionary about Agile Project Management principles, which emphasize teamwork, collaboration, and collective ownership of project outcomes, the Project Manager can help shift the team's mindset towards a more unified approach. This briefing should highlight the value of diverse perspectives and the importance of working together towards common goals, thus addressing the underlying issues contributing to the 'them and us' culture. Encouraging open communication and collaboration as fundamental components of the Agile approach can help break down barriers and build a more cohesive team.