Final answer:
To avoid a close-quarters situation, a significant course change is considered the most effective action since it clearly communicates the vessel's intentions to other vessels nearby.
Step-by-step explanation:
When action to avoid a close-quarters situation is taken, a course change alone may be the most effective action provided that it is a large course change.
This course change should be significant enough to be readily apparent to another vessel observing visually or by radar.
A large course change will clearly indicate your intentions to the other vessel and can help avoid a close-quarters situation more effectively than a series of small course changes, which may be harder to perceive and lead to confusion or indecision in a critical moment.