Option 1: Nuclear powered submarine - A nuclear powered submarine is a type of naval vessel that is powered by a nuclear reactor. It is used for various purposes including reconnaissance, surveillance, and striking enemy targets. However, the Nag is not a nuclear powered submarine, so this option is incorrect.

Option 2: Inter-continental ballistic missile - An inter-continental ballistic missile (ICBM) is a long-range missile that is capable of delivering nuclear weapons to targets thousands of kilometers away. The Nag is not an ICBM, so this option is incorrect.

Option 3: Indigenously built battle tank - A battle tank is an armored vehicle designed for front-line combat. An indigenously built battle tank refers to a tank that is designed and manufactured within the country. However, the Nag is not a battle tank, so this option is incorrect.

Option 4: Anti-tank missile - An anti-tank missile is a missile specifically designed to destroy enemy tanks and armored vehicles. The Nag is indeed an anti-tank missile, as it is an indigenous guided missile system developed by India for use against enemy tanks. Therefore, this is the correct answer.

In conclusion, the correct answer is option 4 - Anti-tank missile.