Which situation is considered a "special circumstance" under the Rules?

A "special circumstance" under the Rules refers to situations that require additional consideration or different responses due to their unique nature. More than two vessels meeting is deemed a special circumstance because it introduces complexity in navigation. When multiple vessels are involved, the challenges of avoiding collisions increase significantly, and the dynamics of right-of-way rules become more intricate.

In typical situations involving only two vessels, the rules are relatively straightforward, as the vessels can clearly ascertain their positions relative to one another. However, when more than two vessels are meeting, it can create confusion regarding which vessel has the right of way and necessitates more careful navigation to avoid collisions.

Other options such as a vessel at anchor or aground do not fit the definition of a "special circumstance" related to vessel meetings. While they may present unique challenges, they do not involve the immediacy of multiple vessel interactions that require heightened situational awareness and decision-making processes that more than two vessels meeting entails.