Amensalism is an association between two species where

1. one species is harmed and other is benefitted

2. one species is harmed and other is unaffected

3. one species is benefitted and the other is unaffected

4. both the species are harmed.

(b) When one species is benefitted and other is harmed, in an association between two species, it is called amensalism. In commensalism, one species is benefitted and the other is unaffected.

The other options are incorrect because While in when one species is harmed and other is benefitted, the relationship is termed as parasitism.