The correct answer is option 2: Think Tank.

1. Statutory Body: A statutory body is an organization created by an act of the legislature with specific powers and functions. NITI Aayog is not a statutory body because it was created by an executive resolution, not an act of the legislature.

2. Think Tank: NITI Aayog is a think tank, which means it is a policy institute or organization that conducts research and provides recommendations on various issues. NITI Aayog`s main role is to provide strategic and technical inputs for policymaking in India.

3. NGO: Non-Governmental Organizations (NGOs) are typically non-profit organizations that operate independently of the government. NITI Aayog is a government organization, so it is not an NGO.

4. Constitutional Body: Constitutional bodies are entities mentioned in the constitution with specific powers and functions. NITI Aayog is not a constitutional body as it does not have its foundation in the constitution.

Therefore, option 2: Think Tank is the correct answer.