Final answer:
The correct answer is option 2. The opening of stomata is controlled by an increase in the solute concentration of guard cells, leading to water influx and turgor pressure increase that opens the stomatal pores, facilitating gas exchange.
Step-by-step explanation:
The opening of stomata involves an increase in the solute concentration of the guard cells. When guard cells increase their solute concentration, water enters the cells through osmosis, leading to an increase in turgor pressure within these cells.
This pressure causes the guard cells to swell, opening the gap between them and forming a stomatal pore through which gases can exchange. Contrarily, a decrease in solute concentration within guard cells would lead to water exiting the cells, causing them to become flaccid and close the stomatal pore. Environmental factors such as light and carbon dioxide concentration are among the stimuli that signal guard cells to modulate their solute levels, thus playing a critical role in regulating water loss through transpiration and allowing for gas exchange necessary for photosynthesis.