In submerged hydrophytes/Vallisneria the functional stomata are found
(a) On the upper leaf surface
(b) On the lower leaf surface
(c) Both leaf surfaces
(d) No where on the plant

Hint: In aquatic plants, the cells that are found on the surface layers of the plant are capable of absorbing water, nutrients, and dissolved gases and with this absorption, the gaseous demand of the plant is fulfilled.

Complete answer:
The plants that are fully submerged are the real water plants or hydrophytes. Hydrophytes (example water ferns) are aquatic plants that have no stomata. In such plants, stomata are either completely absent or even if they are present, they remain vestigial.
The gaseous exchange takes place directly through the body cells of aquatic plants. For this purpose, no other pores such as stoma are needed. Underwater plants also don't have stomata. If stomata were present in plants underwater water would enter through the stomata and fill the plant.
So, the correct answer is, 'No where on the plant.'

Additional Information:
Vallisneria is a genus of aquatic freshwater herb, commonly known as eelgrass, tape grass, or Vallis. The genus is widely distributed in Asia, Africa, Europe, and North America, in tropical and subtropical regions.
Vallisneria is a submerged plant that spreads by runners and often forms high meadows underwater. Leaves emerge from their origin in clusters. The leaves have tips squared, and veins certainly raised. On very long stalks one white female flower rises to the surface of the water. Male flowers emerge, become detached, and float to the surface on short stalks.

Note: Stomata are tiny openings or pores in plant tissue that enable the exchange of gases. Typically, stomata are found in plant leaves but can be present in some stems too. Specialized cells called guard cells surround the stomata and act to open and close the pores of the stomata. Overall, stomata are more frequent on the underside of leaves.