The correct answer is B) running-free. When sailing in a strong wind, a boat that is running-free (sailing with the wind coming from behind) is more susceptible to an accidental jibe than when tacking (sailing into the wind), in irons (head-to-wind), or reaching (sailing across the wind). This is because the boat has less directional stability when running-free, making it more likely to suddenly change direction and experience a dangerous jibe as the boom swings violently across the boat. The other options do not present the same heightened risk of an accidental jibe in strong winds.