Opening and closing of stomata is due to the:

Explanation

The correct answer is that the opening and closing of stomata are driven by changes in the turgor pressure of guard cells (Option B). When guard cells absorb water, they become turgid, causing the stomatal pores to open. Conversely, when they lose water, they become flaccid, leading to the closure of the pores. Option A, concerning hormonal signals, does play a regulatory role but does not directly cause the movement. Options C and D, which mention gaseous exchange and plant respiration, describe processes facilitated by stomata but do not influence their opening and closing directly.