Explanation:

A sunken stomata is a stomata in a small pit, which protects the escaping water vapour from air currents, decreasing water loss from the leaf. Sunken stomata are commonly found in plants in arid environments as one of their adaptations to preserve water. As Nerium is a xerophyte i.e., it cannot take in sufficient amount of needed water, the sunken stomata are present in the lower epidermis to reduce the loss of water by transpiration.