What happens to the guard cells when water flows into them?

The correct option is Stomatal pores open.

Here is a step-by-step explanation of the mechanism behind this process:

1. Structure of Guard Cells: Stomatal pores are microscopic openings on leaf surfaces flanked by two specialized kidney-shaped (or dumbbell-shaped in grasses) cells called guard cells. The cell walls of these guard cells are unevenly thickened: the inner wall facing the pore is thick and rigid, while the outer wall is thin and highly elastic.

2. Water Influx and Turgor Pressure: When plants need to perform photosynthesis, potassium ions ($K^{+}$) are actively pumped into the guard cells. This accumulation of solutes lowers the water potential inside the guard cells, causing water to flow into them from neighboring epidermal cells via osmosis.

3. Bowing Effect: As water enters, the guard cells swell and become turgid. Because the outer walls are thin and flexible, they expand outward more than the thick, rigid inner walls. Cellulose microfibrils in the cell walls are radially oriented, which prevents the cells from expanding in width and instead forces them to lengthen and curve outward.

4. Opening of the Stoma: This outward curving or bowing of the turgid guard cells pulls the inner walls away from each other. As a result, the gap between the guard cells widens, causing the stomatal pore to open and allow gas exchange.