Historically, and for a significant period leading up to and including the early-to-mid 2010s (the probable context of this question), Mauritius was consistently the largest single source country for Foreign Direct Investment (FDI) into India. This was largely due to the favourable Double Taxation Avoidance Agreement (DTAA) between India and Mauritius, which made it an attractive route for investments originating from around the world to enter India. While Singapore's share increased significantly later, and the DTAA with Mauritius was amended effective April 1, 2017, leading to changes in FDI flow patterns, Mauritius held the top spot for many years. Among the given options, Mauritius has historically been the most dominant source country.