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

The 35th constitutional Amendment Act (1975):Terms and Conditions for the Incorporation of Sikkim into the Union of India.

The 37th constitutional Amendment Act (1975):Formation of Arunachal Pradesh legislative assembly.

The 38th constitutional Amendment Act(1975):Enhances the powers of President and Governors to pass ordinances.

The 36th constitutional Amendment Act (1975):Formation of Sikkim as a State within the Indian Union.