Which of the following factors promotes the opening of the stomatal aperture?

The correct option is: "Radial orientation of the cellulose microfibrils in the cell wall of the guard cells".

Stomata are tiny pores found on the surface of leaves and stems that regulate gas exchange and water loss (transpiration) in plants. The opening and closing of the stomatal aperture is controlled by changes in the turgidity of the specialized guard cells surrounding the pore.
When water flows into the guard cells, their turgor pressure increases, causing them to swell and expand.

The cell walls of guard cells are not uniform in thickness or structure:
1. The inner wall (facing the stomatal pore) is thick and inelastic.
2. The outer wall is thin and highly elastic.
Additionally, cellulose microfibrils in the cell walls of the guard cells are arranged radially (pointing outward from the center like spokes of a wheel) rather than longitudinally.

When the guard cells take up water and become turgid, the radial orientation of these cellulose microfibrils prevents the cells from expanding radially (getting wider). Instead, it forces them to expand longitudinally (growing longer).
Because the inner walls are thick and inelastic while the outer walls are thin and stretchable, the expansion causes the guard cells to bow outward. This bowing movement pulls the inner walls away from each other, thereby opening the stomatal aperture.

Therefore, the radial orientation of cellulose microfibrils is crucial because it guides the direction of expansion necessary to open the stomata when turgidity increases.