Explanation:

The National Security Act, 1980 is a preventive detention law. 

• Preventive Detention involves the detainment of a person in order to keep him/her from committing future crimes and/or from escaping future prosecution.
• Article 22 (3) (b) of the Constitution allows for preventive detention and restriction on personal liberty for reasons of state security and public order.
• The NSA empowers the Centre or a State government to detain a person to prevent him from acting in any manner prejudicial to national security.
• The maximum period for which one may be detained is 12 months. But the term can be extended if the government finds fresh evidence. 

Hence only statements 1&2 are correct.