Question

Which one is a water fern?
(a)Azolla
(b)Salvinia
(c)Marsilea
(d)All of the above.

Answer 1

Water fern is a small free-floating water plant that forms dense mats. It was introduced for ornamental use in ponds and aquaria but its introduction into the wild has meant it has spread rapidly throughout.

Complete answer:
Azolla is the genus of seven species of aquatic ferns belonging to Salviniaceae. they're extremely reduced in form and specialized. they appear almost like duckweeds and mosses.
Salvinia, commonly referred to as Water moss is the genus belonging to Salviniaceae. it's a floating water fern. Salvinia is said to other water ferns like Azolla.
Marsilea, commonly referred to as water clover and four-leaf clover may be a genus of roughly 65 species of aquatic ferns belonging to Marsileaceae.
Hence Azolla, Salvinia and marsilea are water ferns.

Additional Information:
The ferns constitute an ancient division of vascular plants, a number of them as old as the Carboniferous (beginning about 358.9 million years ago) and maybe older. Their sort of life cycle, dependent upon spores for dispersal, long preceded the seed-plant life cycle.
Importance to humans:
As a group of plants, ferns aren't of great value. Many species are used as a minor food source and for medicine in various parts of the planet. The ostrich fern (Matteuccia) of northeastern North America is usually eaten, apparently with no ill effect, but the 2 ferns most ordinarily consumed in East Asia (Osmunda and Pteridium) are shown to be strongly carcinogenic. The minute aquatic floating fern (Azolla) has become a valuable plant, especially in Southeast Asia; a cyanobacterium (Anabaena azollae) is usually found in pockets on the leaves of Azolla and helps convert nitrogen to a form usable by other plants (see nitrogen-fixation), thus greatly increasing the productivity of rice paddies where the fern occurs. The best economic value of ferns has been in horticulture, with large nurseries supplying many plants annually for both indoor decoration and outdoor gardens and landscaping. On the negative side, the poisonous bracken (Pteridium aquilinum), which often spoils the grazing value of varied lands, is considered a noxious weed in many countries.
So, the correct answer is, ‘All of the above.’

Note: Water ferns:
-Damp soil to submerged
-Leaves with few leaflets
-Sporangia in sporocarps (water-resistant structures that contain sporangia)
-heterosporous
-e.g. Marsilea, Regnellidium, Pilularia
Floating Ferns:
-Floating plants
-Sporangia in sporocarps
-Heterosporous
-e.g. Salvinia, Azolla