Aquatic ferns are ferns that are adapted to living in or around water. They have specific adaptations that allow them to thrive in wet environments. Among the options given, the correct answer is option C, Salvinia.

Salvinia is a genus of aquatic ferns that belong to the family Salviniaceae. These ferns are commonly known as "water ferns" or "floating ferns" because they float on the surface of the water. They are small, free-floating plants that have specialized leaves and root structures to help them survive in aquatic habitats.

Here are some characteristics and adaptations of Salvinia that make it an aquatic fern:

1. Floating Leaves: Salvinia ferns have floating leaves that are arranged in pairs. The upper surface of the leaves is covered with fine hairs, which help to repel water and keep the leaves dry. This adaptation prevents the fern from becoming waterlogged and promotes buoyancy.

2. Root Structures: Unlike most ferns, Salvinia ferns have reduced or absent roots. Instead, they have specialized root-like structures called "modified leaves" or "rootlets" that hang down into the water. These structures help to anchor the ferns to the substrate and absorb nutrients from the water.

3. Cuticle and Trichomes: The leaves of Salvinia ferns have a thick cuticle, which reduces water loss through evaporation. The leaves are also covered with trichomes, which are tiny hair-like structures that create a waterproof layer. This adaptation helps the ferns to retain moisture and prevents dehydration.

4. Reproductive Structures: Salvinia ferns reproduce through spores, like other ferns. They produce sporocarps, which are small structures that contain the spores. These sporocarps are located at the base of the leaves or in specialized structures called "sporocarps clusters." The spores are released into the water, where they can disperse and grow into new ferns.

Overall, Salvinia is a prime example of an aquatic fern because of its ability to float, specialized leaves and root structures, and adaptations to reduce water loss. These adaptations allow it to thrive in various aquatic habitats, such as ponds, lakes, and slow-moving streams.