In amensalism:

The correct answer is Option (3) → One species is harmed whereas the other is unaffected

Population interactions are marked by their effect on each partner: + (benefit), − (harm) or 0 (no effect). Amensalism is the −/0 interaction.

Option (1) One species is benefitted and the other is neither benefitted nor harmed — Incorrect. This +/0 interaction is commensalism, as with the orchid on a mango branch.

Option (2) One species is benefited whereas the other is unaffected — Incorrect. This is again commensalism (+/0), said in other words.

Option (3) One species is harmed whereas the other is unaffected — Correct. Amensalism is the −/0 interaction. It is one of the population interactions, alongside mutualism (+/+), competition (−/−), predation and parasitism (+/−) and commensalism (+/0).

Option (4) One species is harmed and other is benefitted — Incorrect. This +/− interaction is predation or parasitism. There the predator or parasite gains and the prey or host is harmed; in amensalism neither partner gains.

Note: A similar question was also asked in CUET (UG) 2023 on 31-05-2023 (Slot 3).

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.”