When was Sikkim merged with India?

When was Sikkim merged with India? Correct Answer 1975

The history of Sikkim, begins with contacts between ancient Hindus and Tibetans, followed by the establishment of a Buddhist kingdom or Chogyal in the 17th century. Sikkim emerged as a polity in its own right against a backdrop of incursions from Tibet and Bhutan, during which the kingdom enjoyed varying degrees of independence. In the early 18th century, the British Empire sought to establish trade routes with Tibet, leading Sikkim to fall under British suzerainty until independence in 1947. Initially, Sikkim remained an independent country, until it merged with India in 1975 after a decisive referendum. Many provisions of the Indian constitution had to be altered to accommodate the international treaties and between Sikkim and India.