The correct answer is B) a vessel engaged in search and rescue. Vessels engaged in search and rescue operations, such as Coast Guard vessels, are permitted to display an alternately flashing red and yellow light as a special signal to indicate their status and the nature of their activities. This special light is used to alert other vessels in the area that the search and rescue vessel is engaged in critical operations and may require the right-of-way or special consideration from other vessels. The other answer choices are incorrect because a vessel not under command (A), a vessel restricted in her ability to maneuver (C), and a vessel engaged in river bank protection (D) are not specifically authorized to exhibit the alternately flashing red and yellow light, which is reserved for vessels engaged in search and rescue activities.