A

The potentiometer is an instrument used to compare the electromotive forces (e.m.f.) of two cells.
Step 1: The key principle behind the operation of a potentiometer is that it measures the e.m.f. by balancing it against the potential drop across the length of the wire.
Step 2: A balance point is achieved when the potential difference (p.d.) along the potentiometer wire equals the e.m.f. of the cell being measured.
Step 3: When a jockey is moved along the wire, the potential drop from the positive terminal to the jockey is equal to the e.m.f. of the experimental cell at the balance point.
Therefore, the correct condition for obtaining a balance point is that the e.m.f. of the battery (which provides the current for the potentiometer) becomes equal to the e.m.f. of the cell being measured.
Hence, the correct answer is Option A.