The correct answer is D) when at transit draft. Semisubmersible vessels are designed to operate in different draft configurations, including drilling draft, survival draft, and transit draft. During heavy weather, the vessel is most likely to experience significant structural stresses when at the transit draft, as this configuration typically has a higher center of gravity and less stability compared to the other draft conditions. The reduced stability at transit draft can make the vessel more vulnerable to the forces exerted by heavy seas, leading to increased structural loads and potential damage. The other options are incorrect because: A) Operating at drilling draft, which is the deepest configuration, typically provides the most stability and resistance to heavy weather. B) Deballasting to survival draft, which is a more submerged configuration, also increases the vessel's stability and reduces structural stresses. C) When at survival draft, the vessel is in a more stable and secure configuration to withstand heavy weather.