What is the relationship between water-fern (Azolla) and nitrogen-fixing cyanobacteria (Anabaena) called as?
A. Symbiotic
B. Predation
C. Parasitic
D. Epiphytic

Hint: The cyanobacteria fixes atmospheric nitrogen which is utilized by the fern whereas the fern provides a habitat for the cyanobacteria. They are in symbiotic association with each other.

Complete answer:
Let’s first discuss each of these relationships.
> Symbiotic: It is a close and a long-term biological interaction between two species in which at least one species benefits from the other. For the other species, the relation may be positive, negative or neutral. There are three types: mutualism, commensalism and parasitism.
> Predation: It involves killing of an organism (prey) by another organism (predator) to consume it for energy purposes. Predation provides energy to prolong life. it occurs mostly between two ecological levels. The lower levels are the prey and the higher-level organisms act as the predator. Example- A deer is predated by a lion.
> Parasitic: it is a type of symbiotic relation between two species of plants or animals in which one benefits at the cost of the other organism. It benefiting species is known as the parasite and the other species is known as the host from which the parasite is benefiting. It may or may not involve killing of the host organism.
> Epiphytic: Also known as “air plants”, involves any plant growing with the physical support of another plant or object. Example- mosses, orchids etc.

So, the correct answer is option (A) Symbiotic.

Note:
Azolla forms a symbiotic relationship with Anabaena. The cyanobacterium fixes atmospheric nitrogen, giving the plant access to nitrogen whereas the fern provides a habitat for the cyanobacterium.