(D) Amount of water in guard cells
The guard cells swell when water flows into them, causing the stomatal pore to open, and close when guard cells shrink (lose water).
Source: Chapter 5, Section 5.2.1 Autotrophic Nutrition
The passage explicitly states: "The guard cells swell when water flows into them, causing the stomatal pore to open. Similarly the pore closes if the guard cells shrink." Examiners expect you to recall that it is the turgidity/flaccidity of guard cells (due to water content) that regulates stomatal movement — not CO₂, O₂, or temperature directly.