Step 1: Background of Sikkim’s status
- Initially, Sikkim was a protectorate of India after independence, with the Indian government controlling its defense, external affairs, and communications.
- However, internal unrest and the demand for full integration grew during the early 1970s.
Step 2: Proposal for merger
- In April 1975, the Sikkim Assembly passed a resolution requesting the complete merger of Sikkim with India.
- Following this, the Government of India accepted the request, and Sikkim was admitted as the 22nd state of India.
Step 3: Analyzing the options
- 1965: Too early, merger was not proposed yet.
- 1970: Incorrect, though political movements were emerging, the official resolution was not passed.
- 1975: Correct, year of Sikkim’s formal merger with India.
- 1980: Too late, by then Sikkim was already a state.
Step 4: Conclusion
Thus, the Sikkim Assembly passed the proposal for complete merger with India in 1975.
\[
\boxed{\text{Correct Answer: (A) 1975}}
\]