A

Step 1: Understanding the function of a tangent galvanometer. This device measures the horizontal component of Earth's magnetic field by using the tangent law.
Step 2: The coil of the galvanometer must be aligned with the Earth's magnetic field for accurate measurements.
Step 3: Therefore, the coil should be set in the magnetic meridian, which is the direction of the magnetic field at the location. This alignment ensures that the magnetic field produced by the coil is compared correctly to the Earth's magnetic field.
Hence, the coil must be set in the magnetic meridian or vertically north-south direction.
Therefore, the correct answer is A.