To solve this problem, we need to understand the factors that facilitate the opening of the stomatal aperture. Let's analyze each option:$\newline \bullet$Option 1: Decrease in turgidity of guard cells.$\newline$ - The opening of stomata is facilitated by an increase, not a decrease, in the turgidity of guard cells.$\newline$ - When guard cells become turgid, they swell and the stomatal pore opens.$\newline$ - Therefore, this option is incorrect.$\newline \bullet$Option 2: Radial orientation of cellulose microfibrils in the cell wall of guard cells.$\newline$ - The radial orientation of cellulose microfibrils allows the guard cells to expand lengthwise when they become turgid.$\newline$ - This orientation helps in the opening of the stomatal pore.$\newline$ - Therefore, this option is correct.$\newline \bullet$Option 3: Longitudinal orientation of cellulose microfibrils in the cell wall of guard cells.$\newline$ - Longitudinal orientation would restrict the expansion of guard cells in the direction needed for stomatal opening.$\newline$ - This orientation does not facilitate the opening of the stomatal aperture.$\newline$ - Therefore, this option is incorrect.$\newline \bullet$Option 4: Contraction of outer wall of guard cells.$\newline$ - The outer wall of guard cells does not contract; instead, it becomes more convex as the guard cells swell.$\newline$ - This change in shape helps in the opening of the stomatal pore.$\newline$ - Therefore, this option is incorrect.$\newline$Based on the analysis, the correct option is Option 2: Radial orientation of cellulose microfibrils in the cell wall of guard cells.