Under which one of the following Amendment Acts was Sikkim admitted into the Union of India?

Sikkim was admitted into the Union of India as a full-fledged state through the 36th Constitutional Amendment Act of 1975 [2]. Following the lapse of British paramountcy in 1947, Sikkim initially became a protectorate of India. In 1974, the 35th Amendment Act introduced the status of an 'associate state' for Sikkim, a unique category in the Indian federal scheme [1]. However, this arrangement was short-lived. In 1975, after a referendum, the 36th Amendment was enacted to incorporate Sikkim as the 22nd state of the Indian Union [3]. This amendment repealed the 'associate state' status, omitted the Tenth Schedule (which had been added by the 35th Amendment), and inserted Article 371F to provide special provisions for the state's administration and the protection of its population's rights [2]. The act came into effect on April 26, 1975 [4].

Sources

1. [1] Introduction to the Constitution of India, D. D. Basu (26th ed.). > Chapter 5: NATURE OF THE FEDERAL SYSTEM > NATURE OF THE FEDERAL SYSTEM > p. 64
2. [2] Indian Polity, M. Laxmikanth(7th ed.) > Chapter 77: Special Provisions for Some States > PROVISIONS FOR SIKKIM > p. 562
3. [3] Rajiv Ahir. A Brief History of Modern India (2019 ed.). SPECTRUM. > Chapter 39: After Nehru... > Annexation of Sikkim > p. 682
4. [4] Indian Polity, M. Laxmikanth(7th ed.) > Chapter 6: Union and Its Territory > New States and Union Territories Created After 1956 > p. 55