The correct option is B Inner
The inner wall of guard cells i.e., the wall adjoining the stomatal pore is thicker than the outer wall. Differential thickness of wall and radial micellation of cellulose fibrils are an important structural feature of guard cell. Thick inner wall prevents bulging of inner wall of guard cell toward the stomatal pore, when guard cells become turgid and the thin outer wall is bulged outside, which pulls the inner wall with the help of radial cellulose fibrils, thus opening the stomatal pore.