A

Step 1: Understand the principle of a galvanometer. A galvanometer is an instrument that measures small currents. It has a limited range, which can be extended to measure voltage by using an appropriate resistance.

Step 2: When converting a galvanometer into a voltmeter, we need to ensure that the galvanometer can handle higher voltages without exceeding its maximum current. Therefore, we connect a resistance in series with the galvanometer.

Step 3: The high resistance allows only a small fraction of the voltage to drop across the galvanometer. This way, the majority of the voltage is across the high resistance, which protects the galvanometer from high currents.

Step 4: If we were to connect a low resistance in series, it would allow too much current, potentially damaging the galvanometer. Likewise, connecting resistors in parallel would not be suitable for this conversion because it would not limit the current through the galvanometer appropriately.

Conclusion: Therefore, to effectively convert a galvanometer into a voltmeter, a high resistance must be connected in series with the galvanometer.
Therefore, the correct answer is A.