Aquatic plants are plants that grow and thrive in water or in waterlogged environments. They are specially adapted to live in these conditions and play an important role in maintaining the balance of aquatic ecosystems. There are various types of aquatic plants, including those that grow partially submerged in water and those that grow completely submerged.

The correct answer to the given question is option A, Hydrilla. Hydrilla is an aquatic plant that grows completely submerged in water. It is a rooted plant that can be found in freshwater environments such as lakes, ponds, and slow-moving rivers.

Below are the characteristics and features of Hydrilla that make it a completely submerged aquatic plant:

1. Morphology:
- Hydrilla has long and slender stems with whorls of leaves arranged around them.
- The leaves are strap-shaped and have finely serrated edges.
- The plant can grow up to several meters in length.

2. Adaptations:
- Hydrilla has adapted to survive in submerged conditions by having thin, flexible leaves that can withstand water pressure.
- It has small pores called stomata on the leaves that allow gas exchange to occur underwater.
- The finely serrated leaf edges help reduce drag and allow water to flow smoothly over the plant.

3. Reproduction:
- Hydrilla can reproduce through vegetative propagation, where new plants are formed from fragments of the parent plant.
- It can also reproduce through sexual reproduction, producing flowers and seeds.

4. Ecological Impacts:
- Hydrilla can form dense mats on the water surface, reducing the amount of sunlight and oxygen available to other aquatic organisms.
- It can outcompete native plant species and disrupt the natural balance of the ecosystem.

In contrast, the other options mentioned in the question are not completely submerged aquatic plants:

- Water hyacinth (option B) is a free-floating aquatic plant with roots that dangle in the water, but its leaves and flowers are mostly above the water surface.
- Water lily (option C) is a partially submerged aquatic plant with floating leaves and flowers that rise above the water.
- Lotus (option D) is a partially submerged aquatic plant with leaves and flowers that float on the water surface.

Therefore, the correct answer is option A, Hydrilla, as it is the only plant mentioned that grows completely submerged in water.