An a. c. source is connected across a pure inductor. Which one of the following figures shows the correct phase relation between the current and e.m.f.?

In an ideal inductor driven by an alternating current source, the voltage leads the current by a phase difference of $\frac{\pi}{2}$ radians (or $90^\circ$), or equivalently, the current lags the voltage by $90^\circ$.

Analysis by elimination:

Option (A) depicts a $180^\circ$ phase shift, which is incorrect.
Option (B) shows the voltage along the positive x-axis and the current along the negative y-axis; hence, voltage leads current by $90^\circ$.
Option (C) shows zero phase difference, characteristic of a resistive circuit.
Option (D) illustrates current leading voltage by $90^\circ$, typical of a capacitive circuit.

Only option (B) correctly represents the phase relationship for a pure inductor.

The correct choice is $\boxed{B}$.