B.N. Rau prepared a series of background papers on political systems in other countries because he was appointed as the Constitutional Advisor to the Constituent Assembly of India. His role was crucial in providing valuable insights and expertise in the process of drafting the Indian Constitution. To ensure that the Indian Constitution was well-informed and drew from the best practices of political systems around the world, B.N. Rau conducted a close study of political systems in other countries and prepared these background papers as a valuable resource for the assembly's members. This helped in shaping the provisions of the Indian Constitution and ensuring that it was well-informed and comprehensive in its approach.

So ,the correct option is [b].