Transpiration is loss of water in vapour form through the stomatal pores (Label C).
The stomatal pore is regulated by guard cells (Label D).
The stomatal pore, guard cells and surrounding 3 to 4 thin walled subsidiary cells (Label B), together constitute the stomatal apparatus.
The stomatal apparatus is present in the epidermal layer (Label A). The guard cells contain stomata and in this respect are different from the epidermal cells.