The correct answer is D) "Passing 50°". The phrase "How's your head?" is a common term used by pilots to inquire about the heading of the vessel. The appropriate response is to state the current heading, in this case, "Passing 50°", which indicates that the vessel's heading is passing through 50 degrees. The other options are incorrect because: A) "Eased to 10° rudder" is a command to adjust the rudder, not a response to the heading inquiry. B) "Steady" is a vague response that does not provide the specific heading information requested. C) "Checked" simply acknowledges the inquiry without providing the actual heading.