Step 1: Understanding the question.
The question asks for the capital city of the Indian state of Sikkim.
Step 2: Explanation.
Sikkim is a small Himalayan state in northeastern India. Its capital city is Gangtok, which is also the largest city in the state. It serves as the administrative, cultural, and economic center of Sikkim.
Step 3: Conclusion.
Hence, the correct answer is (D) Gangtok.