The World Network of Biosphere Reserves is a dynamic list. As of recent times, all four options (Simlipal, Sunderban, Gulf of Mannar, and Nilgiri) are included in the UNESCO list. However, their inclusion dates vary: Nilgiri (2000), Gulf of Mannar (2001), Sunderban (2001), Simlipal (2009). If this question was asked before 2009, Simlipal would be the correct answer as it was a declared biosphere reserve by India but not yet part of the international network.