The question is about a robot named "CAPTAIN ARJUN" and its purpose in screening passengers. To find the correct answer, let's analyze the options:

• Train: CAPTAIN ARJUN is indeed designed for screening passengers who board the train. The robot was launched by the Indian Railways to enhance the safety measures at railway stations. It is equipped with advanced features like thermal screening, motion sensors, and face recognition to identify potential security threats and ensure the safety and health of the passengers.
• Aeroplane: Although screening measures are crucial for air travel, CAPTAIN ARJUN is specifically developed for railway stations, not airports.
• Ship: The security measures on ships are generally handled by other means, and "CAPTAIN ARJUN" is not designed for marine use.
• Bus: Similar to airports, buses might have security protocols, but CAPTAIN ARJUN is not associated with bus boarding procedures.

Thus, the correct answer is that CAPTAIN ARJUN is a robot launched to screen passengers while they board trains.