Which of the following is categorized as an aircraft?

A helicopter is categorized as an aircraft because it is specifically designed and engineered for flight, utilizing rotors for lift and propulsion. Aircraft, by definition, encompass any vehicle capable of atmospheric flight, which includes fixed-wing planes and rotary-wing vehicles like helicopters. Helicopters operate within the same airspace as airplanes, subject to similar regulations concerning safety, navigation, and air traffic control.

The other options do not fall under the category of aircraft due to their design and purpose. A yacht is a watercraft intended for navigation on water, a submarine operates underwater and is designed for marine environments, while a train runs on tracks for ground transport. None of these vehicles are capable of flight and therefore do not meet the criteria for classification as an aircraft.