Sub-merged hydrophytes are those plants which live completely inside the water, so there is no need of transpiration that's why these plants do not have stomata, e.g. Utricularia, Ceratophyllum.

Stomata are small pores present on the surface of leaves and stems of plants. They are responsible for gas exchange, allowing the plant to take in carbon dioxide for photosynthesis and release oxygen and water vapor. However, not all plants have stomata.

The correct answer is option 'D': submerged hydrophytes. Submerged hydrophytes are aquatic plants that grow completely submerged in water. They do not require stomata because they obtain carbon dioxide directly from the water.

To understand why submerged hydrophytes do not have stomata, let's take a closer look at the different types of plants mentioned in the options:

a) Hydrophytes: These are plants that are adapted to living in aquatic environments, such as ponds, lakes, and swamps. They have stomata on the upper surface of their leaves, allowing them to exchange gases with the surrounding air. This adaptation helps them to obtain carbon dioxide needed for photosynthesis.

b) Mesophytes: Mesophytes are plants that grow in moderate climates with average water availability. They have stomata on both the upper and lower surfaces of their leaves. This arrangement of stomata helps to regulate water loss through transpiration while allowing for gas exchange.

c) Xerophytes: Xerophytes are plants that are adapted to arid or desert environments, where water availability is limited. They have specialized adaptations to minimize water loss, such as reduced stomata density or the presence of stomata in pits or grooves. These adaptations help to conserve water while still allowing for gas exchange.

d) Submerged hydrophytes: Submerged hydrophytes are fully aquatic plants that grow completely submerged in water. They do not require stomata because they obtain carbon dioxide directly from the water through diffusion. The water surrounding them is rich in dissolved carbon dioxide, which can freely enter the plant's cells.

In summary, while most plants have stomata to facilitate gas exchange, submerged hydrophytes do not require stomata as they obtain carbon dioxide directly from the water.