Explanation:

- Nagaland: Nagaland, a northeastern state in India, has a functional airport in Dimapur. It has been operational for many years, connecting Nagaland with other parts of the country.

- Sikkim: Prior to September 2018, Sikkim was indeed the only state in India without a functional airport. The Pakyong Airport, which officially opened in September 2018, was the first ever in the state, improving connectivity to this mountainous region.

- Assam: Assam has several operational airports, including the Lokpriya Gopinath Bordoloi International Airport in Guwahati.

- Mizoram: Mizoram has a functional airport located in Lengpui, which serves the capital city of Aizawl.

? Correct Answer: Sikkim