Physics - Electric Field

Since the charge involved is a point charge, the electirc field lines run along in a radial direction, with circular equipotential lines centred at the point charge.

As such, by definition of equipotential line, the potential V along the line is constant.

As the test charge moves along the equipotential line, its distances from the point chrage remain the same. Because the magnitude of electric field varies with distance from the point charge, a constant distance indicates a constant magnitude of electric field (though the direction of magnetic field line varies).

The answer is thus option B