Sunken stomata occurs in
(a) Mango
(b) Guava
(c) Hydrilla
(d) Nerium

Hint: The presence of sunken stomata is a characteristic feature of xerophytic plants to prevent water loss through respiration. Mango and Guava are mesophytic plants, whereas the hydrilla is a hydrophytic plant.

Correct step by step answer:
Sunken stomata are found in Nerium to control the rate of transpiration. In xerophytes, the stomata are sunken as they are located in a cup- shaped depression (e.g., Nerium) . It is an adaptation to reduce the loss of water in xerophytic plants. Nerium,a xerophytic plant belonging to the family Apocynaceae is a shrub or small tree, cultivated worldwide in temperate and subtropical areas as an ornamental and landscaping plant.
- Nerium is tolerant of both drought and inundation, but not to prolonged frost. The fruit is a long narrow pair of follicles, which splits open at maturity to release numerous downy seeds.
- Stomata are the openings found on the epidermis of leaves and young stems. Each stoma is surrounded by guard cells, which are two specialized epidermal cells. Guard cells are bordered by one or more modified epidermal cells, called subsidiary cells or accessory cells. Usually, the stomata are placed at the same level as the adjoining epidermal cells (e.g., Helianthus, Mangifera).
- Mango and Guava are mesophytic plants. Mesophytes are terrestrial plants that are neither adapted to particularly dry nor particularly wet environments.
- Hydrilla is a hydrophytic plant adapted to live in an aquatic environment. Since they have plenty of water around hydrophytes often have a reduced cuticle and fewer stomata than other plants.
So, the correct answer is, ‘(d) Nerium’.

Note:
- Several compounds in Nerium exhibit toxicity, and it has historically been considered a poisonous plant.
- Ingestion of larger amounts may cause nausea, vomiting, excess salivation, and abdominal pain.
- Prolonged contact with the sap may cause skin irritation, eye inflammation, and dermatitis.