The correct answer is A) deviation and variation. To find a magnetic compass course from a true course, you must apply both deviation and variation. Deviation refers to the magnetic error caused by the ship's metal and magnetic fields, while variation refers to the difference between true north and magnetic north. Both of these factors must be accounted for to convert between true and magnetic courses. The other options are incorrect because: B) deviation alone is not enough, as variation must also be applied; C) variation alone does not account for the magnetic error caused by the ship itself; and D) magnetic anomalies are local disturbances that affect the compass, but do not need to be applied to convert between true and magnetic courses.