(C) Guard cells
Guard cells control the opening and closing of the stomatal pore by changing their water content, causing them to swell or shrink.
Guard cells are specialised cells surrounding each stoma. They regulate stomatal movement by changing their turgidity. Epidermal cells form the outer covering; chloroplasts are organelles involved in photosynthesis. Stomata is the pore itself, not the controlling structure. Examiners expect students to distinguish between the pore (stomata) and the cells that regulate it (guard cells).