Relative Humidity is considered as the most important factor in regulating the closing and opening of stomata. Since it indirectly controls transpiration and stomata's main function is transpiration.
The best and most correct answer among the choices provided by the question is the second choice "humidity."
Opening and closure<span> of the </span>stomatal<span> pore is mediated by changes in the turgor pressure of the two guard cells. The turgor pressure of guard cells is </span>controlled<span>by movements of large quantities of ions and sugars into and out of the guard cells.</span>
I hope my answer has come to your help. God bless and have a nice day ahead!
