Concept:A base line needs a known bearing; a magnetic compass gives magnetic direction.Explanation:In surveying, a base line is a fixed reference line. To set its direction, the surveyor must find its bearing. A magnetic compass measures the magnetic bearing of a line. The other instruments have different roles: an abney level measures slopes, a ranging pole marks stations, and a plumb bob tests vertical alignment. None of them determine horizontal direction. Thus the magnetic compass is the tool used to locate the direction of a base line.Answer:D. magnetic compass