Which shape of topmark signifies a Lateral Port Hand Mark in Region A?

The correct choice is a single red cylinder, which serves as the topmark that signifies a Lateral Port Hand Mark in Region A of the IALA Maritime Buoyage System. The specific use of a red cylinder for this purpose is part of the standardized buoyage system that aids in maritime navigation.

In this system, lateral marks are crucial for indicating the sides of navigable channels, with colors red and green representing the port and starboard sides respectively. The choice of shape and color is designed to be easily recognizable to mariners, ensuring safer passage by clearly indicating the correct side to navigate on when approaching a harbor or channel.

In this context, the presence of a red shape distinctly identifies the port hand mark, which indicates the left side of a channel when a vessel is proceeding upstream. The use of a cylinder as a topmark communicates this important navigational information effectively.