During your oral exam you are asked to explain the concept of "free surface effect" and why it is relevant to a yacht with partially filled fuel or water tanks.

A. It describes the wave pattern created by the vessel's hull at speed

B. It refers to the movement of liquid within a partially filled tank as the vessel rolls, which shifts the centre of gravity and reduces the vessel's effective stability (GM)

C. It only affects vessels carrying bulk cargo and is irrelevant to yachts

D. It is the term for water entering the vessel through an open porthole