Explanation:

- Pakyong: This is the first greenfield airport in the northeastern region of India, located in Sikkim. Greenfield airports are constructed on undeveloped sites from scratch.

- Lengpui: This is an airport in Mizoram but not a greenfield project. It was constructed before Pakyong.

- Paro: This airport is located in Bhutan, not in India. It is not relevant to the question.

- Bagdogra International: Situated in West Bengal, it is an important airport but not a greenfield project.