In a mutualistic relationship, which of the following is correct?

The correct answer is Option (2) → Both species derive benefits from the interaction.

An interaction between two species can help (+), harm (–) or not affect (0) each of them. In mutualism (+, +) both species gain. Lichens (a fungus with algae or cyanobacteria), mycorrhizae (fungi with the roots of higher plants) and the fig tree with its pollinating wasp are the book’s examples.

Option (1) One species benefits while the other is neither harmed nor helped. — Incorrect. One species gaining while the other is neither helped nor harmed (+, 0) is commensalism, as with an orchid growing on a mango branch.

Option (2) Both species derive benefits from the interaction. — Correct. Both partners gain. In a lichen, for example, the fungus gets food from the alga or cyanobacterium, and the fungus in turn gives shelter and absorbs water and minerals for its partner.

Option (3) One species benefits at the expense of the other. — Incorrect. One species gaining at the cost of the other (+, –) describes predation and parasitism, where the prey or the host is harmed.

Option (4) Both species suffer reduced fitness. — Incorrect. When both species lose (–, –), the interaction is competition, in which the fitness of each species is lowered in the presence of the other.

NCERT Extract:
Population Interactions – “… Both the species benefit in mutualism and both lose in competition in their interactions with each other. In both parasitism and predation only one species benefits (parasite and predator, respectively) and the interaction is detrimental to the other species (host and prey, respectively). The interaction where one species is benefitted and the other is neither benefitted nor harmed is called commensalism.”
Population Interactions – “… (v) Mutualism: This interaction confers benefits on both the interacting species. Lichens represent an intimate mutualistic relationship between a fungus and photosynthesising algae or cyanobacteria. Similarly, the mycorrhizae are associations between fungi and the roots of higher plants.”