The correct option is A guard cells
The stomatal apparatus is a part of the epidermal tissue system that helps in exchange of gases and also transpiration.
The part labelled as A is epidermal cell, B is subsidiary cell, C is guard cell and D is stomatal pore.
The guard cells are specialized cells present in epidermis of leaves. The two guard cells enclose a pore in between them, known as stomatal pore. The epidermal cells located next to guard cells are called subsidiary cells.