The correct answer is option 4: Nuclear-powered ballistic missile submarine.

1. Multi-barrel rocket launcher: This refers to a weapon system that can launch multiple rockets simultaneously. It is typically used to provide a high volume of firepower in a short amount of time, but it is not the same as Arihant.

2. Airborne Early Warning and Control System (AEW&C): This refers to an aircraft that is equipped with radar and other sensors to detect and track enemy aircraft. AEW&C systems are used for surveillance and command and control purposes, and Arihant is not an AEW&C system.

3. Unmanned Combat Aerial Vehicle (UCAV): This refers to a drone or unmanned aircraft that is capable of carrying out combat missions. UCAVs are often used for reconnaissance, surveillance, and strike missions, but Arihant is not a UCAV.

4. Nuclear-powered ballistic missile submarine: This is the correct answer. Arihant is a class of Indian Navy submarines that are powered by nuclear reactors and are capable of carrying ballistic missiles. These submarines provide a significant deterrent capability to India`s naval forces.

In conclusion, Arihant is a nuclear-powered ballistic missile submarine, not a multi-barrel rocket launcher, AEW