Opening and closing of stomata is due to :

Option (b) is correct because the physical mechanism of stomatal movement is driven by the turgidity of the guard cells, which depends on the movement of water in and out of them. Option (a) is incorrect because high gaseous pressure inside the cells does not mechanically drive the opening/closing. Option (c) is incorrect because while light acts as an environmental stimulus, the direct physical cause of the movement is water flow. Option (d) is incorrect because the diffusion of $\text{CO}_2$ is a consequence of the stomata being open, not the mechanism that causes them to open or close.