For interspecific interactions, a ‘+’ sign is assigned for a benefici…

The correct answer is Option (3) → Commensalism : (+) and (0)

For an interaction between two species, each species gets a sign: (+) if it gains, (−) if it is harmed and (0) if it is not affected. Mutualism is (+, +), competition (−, −), predation and parasitism (+, −), commensalism (+, 0) and amensalism (−, 0).

Option (1) Amensalism : (+) and (−) — Incorrect. In amensalism one species is harmed and the other is unaffected, so the signs are (−) and (0). The pair (+) and (−) belongs to predation and parasitism.

Option (2) Competition : (−) and (0) — Incorrect. In competition both species lose, so the signs are (−) and (−). The pair (−) and (0) is amensalism.

Option (3) Commensalism : (+) and (0) — Correct. One species is benefitted (+) and the other is neither benefitted nor harmed (0).

Option (4) Parasitism : (−) and (−) — Incorrect. In parasitism the parasite benefits and the host is harmed, so the signs are (+) and (−). The pair (−) and (−) is competition.

Note: Six interactions with two signs each, and Table 11.1 holds all of them in fewer words than this Note. Learn it as a grid of signs and a question like this one takes ten seconds.

NCERT Extract:
Population Interactions – “… Assigning a ‘+’ sign for beneficial interaction, ‘-’ sign for detrimental and 0 for neutral interaction, let us look at all the possible outcomes of interspecific interactions (Table11.1). 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.”