Openings between cells create pores in the leaf epidermis that play a critical role in photosynthesis by allowing diffusion of atmospheric CO2 into a leaf's interior.

Guard

Explanation:

The opening between guard (pores) cells is known as stomata. The stomata helps in diffusion of atmospheric CO2 into leaf's interior and also involved in loss of water by plant leaves through transpiration. The opening and closing of stomata is controlled by guard cells.

Guard cells control the opening and closing of stomata by becoming turgid or flaccid because of potassium ions. The water potential decreases by increasing the solute concentration and water always flow from higher potential to lower potential. Remember maximum water potential is zero.

When potassium ions concentration increases inside the guard cells, water follow the potassium ions and guard cells become turgid which cause the opening of stomata. On the other side when the potassium ions concentration decreases inside the guard cells the water leaves the guard cells and guard cells become flaccid causing closing of stomata.

well, that's easy; the answer's c, "it reduces contamination of the water and substrate of the estuary."