The correct answer is B) broad on the starboard bow. A vessel spotted at 45° relative to the observer's vessel is considered to be "broad on the starboard bow". This is because the relative bearing of 45° falls within the range of 22.5° to 67.5° off the starboard bow, which is the accepted definition of "broad on the starboard bow" according to standard maritime terminology and navigation practices. The other options are incorrect because: A) 4 points forward of starboard bow (315°) is not the same as 45° relative; C) 4 points abaft the starboard beam (135°) is not the same as 45° relative; and D) on the starboard beam (90°) is not the same as 45° relative.