Which of the following option is correct for ammensalism?

The correct answer is Option (2) → One species is harmed and other remains unaffected

In amensalism one species is harmed while the other is unaffected (−, 0).

Option (1) One species is harmed and other is benefitted — Incorrect. One harmed and the other benefitted (−, +) is predation or parasitism.

Option (2) One species is harmed and other remains unaffected — Correct. In amensalism one species is harmed while the other is neither harmed nor benefited (– / 0). This is the definition NCERT gives for this interaction.

Option (3) One species is benefitted and other remains unaffected — Incorrect. One benefitted and the other unaffected (+, 0) is commensalism.

Option (4) Both the species are harmed — Incorrect. When both species are harmed (−, −), the interaction is competition; NCERT says both lose in competition. In amensalism only one species is harmed and the other is unaffected, so this does not fit.

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. In amensalism on the other hand one species is harmed whereas the other is unaffected.”