Most effective light for stomatal opening is

The correct option is Blue.

Stomatal opening is regulated by light, which stimulates the guard cells surrounding the stomatal pore. While photosynthesis in guard cell chloroplasts occurs under both red and blue light (leading to solute accumulation and subsequent opening), guard cells possess a highly sensitive, specific photoreceptor system that responds specifically to blue light.

Blue light is perceived by specific phototropins (blue light photoreceptors) located on the plasma membrane of guard cells. When blue light hits these photoreceptors, it triggers a signaling cascade that activates $H^{+}$-ATPase pumps (proton pumps) in the cell membrane.
This activation leads to the extrusion of protons ($H^{+}$) out of the guard cells, creating an electrochemical gradient.

In response to this electrical gradient, potassium ions ($K^{+}$) and chloride ions (${\mathrm{Cl}}^{-}$) flow into the guard cells from surrounding cells. The high concentration of these solutes lowers the water potential inside the guard cells, causing water to enter them via osmosis. The resulting turgor pressure causes the guard cells to swell and bow outward, opening the stomatal pore. Because of this specialized blue-light-activated proton pump mechanism, blue light is significantly more effective at inducing stomatal opening than any other wavelength of light.