The correct answer is C) 180°. The relative bearing of an object dead astern (directly behind the vessel) is 180° because relative bearings are measured clockwise from the vessel's bow (front) to the object. When an object is directly behind the vessel, its relative bearing is 180°. The other options are incorrect because: A) 090° would be off to the starboard (right) side of the vessel, B) 000° would be directly ahead of the vessel, and D) 270° would be off to the port (left) side of the vessel. None of these options accurately represent the relative bearing of an object that is dead astern of the vessel.