A

Step 1: Understand the potentiometer setup: The potentiometer is designed to measure the e.m.f. of a cell and works based on the principle that the potential difference across a uniform wire is directly proportional to its length.
Step 2: Analyze the problem: The galvanometer shows deflection in one direction, indicating there is a potential difference between the cell E and the emf of the battery V. The continuous decrease in deflection implies that as point P moves from X to Y, the potential difference decreases, meaning that either the voltage of E is less than V or, more likely, the resistance R is too high to allow a proper balance to be achieved.
Step 3: To obtain a balance point, the resistance R must be adjusted. Decreasing the resistance R increases the current flowing through the circuit, allowing for a greater potential difference along the potentiometer wire. This would help achieve a balance more quickly and effectively.
Therefore, the correct action is to: decrease the resistance R (Option A).