C

Step 1: Understand what a tangent galvanometer is. A tangent galvanometer is a device used to measure the strength of an electric current. It consists of a coil of wire that is mounted on a pivot and can rotate in a horizontal plane.
Step 2: The tangent galvanometer operates on the principle that a magnetic field is created around the coil when current flows through it. The magnetic field of the coil interacts with the Earth's magnetic field.
Step 3: The angle through which the coil rotates, relative to the magnetic north, is directly proportional to the strength of the current flowing through the coil. This relationship allows the galvanometer to provide a measurement of current based on the angle of deflection.
Conclusion: Therefore, a tangent galvanometer is used primarily to measure current.