Concept: This question tests the understanding of the ionic basis of repolarisation in a neuron during an action potential.

Why (D) is correct: Repolarisation is the phase where the membrane potential returns to its resting negative state after depolarisation. This occurs primarily due to the efflux (outflow) of potassium ions (${\mathrm{K}}^{+}$) from the intracellular fluid to the extracellular fluid through voltage-gated ${\mathrm{K}}^{+}$ channels. This outflow of positive charge makes the inside of the membrane more negative.

Why other options are wrong:

• (A) Influx of ${\mathrm{Na}}^{+}$: Influx of ${\mathrm{Na}}^{+}$ causes depolarisation, not repolarisation.
• (B) Influx of ${\mathrm{K}}^{+}$: Influx of ${\mathrm{K}}^{+}$ would make the inside more positive, which is not repolarisation.
• (C) Efflux of ${\mathrm{Na}}^{+}$: Efflux of ${\mathrm{Na}}^{+}$ is primarily handled by the ${\mathrm{Na}}^{+}/{\mathrm{K}}^{+}$ pump to restore ion gradients after an action potential, but it's not the direct cause of repolarisation.

Common Mistake: Students often confuse the roles of ${\mathrm{Na}}^{+}$ and ${\mathrm{K}}^{+}$ in depolarisation and repolarisation, or the direction of their movement (influx vs. efflux).

NEET Tip: Remember the sequence: Depolarisation = Na⁺ Influx; Repolarisation = K⁺ Outflux. Think of 'Na-in' for 'up' and 'K-out' for 'down'.

Correct Answer: (D)