The correct option is
B Decrease transpiration
A sunken stomata is a stomata in a small pit, i.e. stomata are embedded into the leaf layers rather than on the surface of the leaves. This helps in protecting against the escaping water vapor from air currents thereby decreasing water loss from the leaf and hence decreasing transpiration. Because of this reason, sunken stomata are one of the adaptations to preserve water. Sunken stomata are commonly found in plants in the desert and arid environments.
Thus, the correct answer is option (B).