Rustom-2, also known as TAPAS-BH-201, is a type of Unmanned Aerial Vehicle (UAV). This means it is an aircraft that is operated remotely, without a human pilot onboard.

Option 1, Fighter aircraft, is incorrect because Rustom-2 is not designed for air-to-air combat. Fighter aircrafts usually carry weapons for the purpose of destroying enemy aircrafts, which is not the case with Rustom-2.

Option 2, Helicopter, is incorrect because Rustom-2 is not a helicopter. A helicopter is a type of rotorcraft in which lift and thrust are supplied by one or more engine-driven rotors.

Option 3, Transport aircraft, is also incorrect as Rustom-2 is not used for transporting goods or passengers. Transport aircrafts are typically larger and capable of longer flights compared to other types of aircrafts.

Option 4, Unmanned aerial vehicle (UAV), is the correct answer because Rustom-2 is an unmanned vehicle which can fly without a human onboard and can be remotely operated. These are often used for missions that are too dangerous for manned aircraft.