Answer:

Stomata are tiny pore present on the epidermal layer of the leaves, which is meant for gaseous exchange called transpiration. Each stomatal apperture is guarded by a pair of pea-shaped specialized cells called chloroplast-containing guard cells, these are the cells which regulates the opening and closing of stomata. The guard cells are surrounded by subsidiary cells. The inner wall of guard cells is thick whereas the outer wall is thin in structure.

So, the correct answer is option D.