Concept:The angle of dip is measured using an instrument that allows a magnetic needle to align with the Earth's magnetic field.Explanation:A dip circle contains a magnetic needle that can move freely in the vertical plane.When aligned with the magnetic meridian, the needle settles at an angle to the horizontal.This angle is the angle of dip.Therefore, the instrument used to measure it is the dip circle.Other options like dip meter or dip variation are not the correct names for this device.Answer:A. Dip circle