Explanation:

The Defence Research and Development Organisation (DRDO) has conducted the final development test of Air Independent Propulsion (AIP) System, crucial for diesel electric submarines (not for nuclear submarines).

• The AIP system based submarines will be required to surface much less frequently, thus increasing their lethality and stealth multifold. 
• The AIP system based conventional submarines can stay underwater for a longer duration.
• Diesel-electric submarines require them to come to the surface frequently to charge their batteries, thus their underwater endurance time is less.
• However, ‘Air-independent’ propulsion technology helps to make the diesel generator less dependent on surface air, thus reducing the need to come on surface frequently.

Hence only statement 2nd is correct.