This is one of the most recent and valid explanations for stomatal movements

The correct option is Potassium efflux and the influx.

Stomatal movements (the opening and closing of stomata) are primarily regulated by turgor changes in the guard cells. Over the years, several theories have been proposed to explain these turgor changes, but the most widely accepted and modern explanation is the active potassium ($K^{+}$) transport theory (also known as the Levitt hypothesis).

Here is a step-by-step breakdown of how potassium efflux and influx regulate stomatal movements:
1. Stomatal Opening (Potassium Influx):
During the day or in the presence of light, starch in the guard cells is converted into malic acid. This malic acid dissociates into malate ions and hydrogen ions ($H^{+}$). The hydrogen ions are actively pumped out of the guard cells, and in exchange, potassium ions ($K^{+}$) are actively taken in (influx) from the surrounding accessory/subsidiary cells. Cl^- ions are also taken in to maintain electrical neutrality. The accumulation of $K^{+}$ and malate ions inside the guard cells increases the osmotic concentration, causing water to enter the guard cells from adjacent cells via endosmosis. As a result, the guard cells become turgid, stretch outward, and the stomatal pore opens.
2. Stomatal Closing (Potassium Efflux):
In the dark or under water-stress conditions (often mediated by the plant hormone abscisic acid, ABA), the active transport of ions is reversed. Potassium ions ($K^{+}$) move out (efflux) of the guard cells into the surrounding subsidiary cells. This reduces the osmotic concentration inside the guard cells, causing water to move out of them via exosmosis. As the guard cells lose water, they become flaccid, and the stomatal pore closes.

Therefore, the continuous regulation of potassium efflux and influx remains the most recent and valid explanation for stomatal movements.