The opening of stomata is thought to involve the opening of stomata is thought to involve an increase in the solute concentration of the guard cells decreased turgor pressure in guard cells active transport of water out of the guard cells movement of k+ from the guard cells

The answer is 'an increase in the solute concentration of the guard cells'. This makes the cell hypertonic to the neighbouring cells and extracellular fluid. Thus water flows into the guard cell by osmosis. When turgid, the stomata pore between the two bean-shaped guard cells, increases in size.