Explanation:

Sikkim was made a part of India after the 35th Constitutional Amendment and was made a co-state. 
History of Sikkim
6 May 1975: Sikkim became a part of India
NSA Ajit Doval had played a prominent role in Sikkim’s merger with India.
In 2003, China acknowledged Sikkim as a part of India.
In 1642, Chogyal dynasty was established in Sikkim.
Sikkim is the only state to have the status of a co-state in India.
Sikkim is the least populous state in India.
The third highest peak in the world, Kanchenjunga is in Sikkim.
The only road to China, Nathula Pass is in Sikkim.