Which aircraft has the highest seating capacity among the listed options?

The A321 is known for its higher seating capacity compared to the other aircraft listed. It is designed to accommodate a larger number of passengers while still maintaining operational efficiency. Typically, the A321 can seat more passengers than the A319 and A320, primarily due to its longer fuselage, which allows for additional rows of seats. The A300, while a wide-body aircraft that can carry a significant number of passengers, is not included in the same category as the others, which are narrow-body jets predominantly used for short to medium-haul flights. Therefore, when determining which aircraft has the highest seating capacity among the choices, the A321 clearly stands out as the correct answer.