A

Step 1: Understand the function of a tangent galvanometer. It is an instrument that measures the current flowing through it by the torque exerted on a coil due to the earth's magnetic field.
Step 2: When connected in series with a known standard resistance, the galvanometer measures the current directly. This setup effectively allows it to function as an ammeter, which measures the flow of electric current in amperes.
Step 3: A tangent galvanometer cannot directly measure voltage; hence, it cannot function as a voltmeter.
Conclusion: Therefore, the correct option is A, as it can be used as an ammeter.