When a.c. source is connected across a resistance, the correct...

In a purely resistive AC circuit, the alternating current and the alternating voltage (emf) are always in phase. This means the phase difference between them is zero.

Therefore, the phasors representing the current ($i_R$) and the emf ($e_R$) must be parallel and point in the same direction.

Looking at the given figures:
(A) shows current leading emf by $90^{\circ}$.
(B) shows current lagging emf by $90^{\circ}$.
(C) shows current and emf out of phase by $180^{\circ}$.
(D) shows current and emf in phase.

Thus, figure (D) correctly represents the phasor relation for a resistance.

Answer: (D)