Which of the following is an example of a competitive interaction?

Balanus and Chthamalus barnacles

The correct answer is Option (4) - Balanus and Chthamalus barnacles

Competition: It is generally believed that competition occurs when closely related species compete for the same resources that are limiting, but this is not entirely true. Totally unrelated species could also compete for the same resource. when resources are limited the competitively superior species will eventually eliminate the other species, but evidence for such competitive exclusion occurring in nature is not always conclusive. Strong and persuasive circumstantial evidence does exist however in some cases. A species whose distribution is restricted to a small geographical area because of the presence of a competitively superior species, is found to expand its distributional range dramatically when the competing species is experimentally removed. Connell’s elegant field experiments showed that on the rocky sea coasts of Scotland, the larger and competitively superior barnacle Balanus dominates the intertidal area, and excludes the smaller barnacle Chathamalus from that zone.