If an ammeter is to be used in place of a galvanometer then we must connect

A galvanometer must be converted into an ammeter by connecting a low resistance (shunt resistance $R_s$) in parallel. This configuration yields a low equivalent resistance, ensuring minimal disturbance to the circuit current. Most current flows through the shunt, protecting the galvanometer while allowing proportional deflection for measurement.

Series connection of either high or low resistance increases total resistance, making the device unsuitable as an ammeter. Parallel connection of a high resistance fails to achieve the necessary low resistance.

The correct choice is connecting a low resistance in parallel.

Final answer: $\boxed{A}$