How can the spread of prickly pear cactus (an exotic species) be controlled in Australia?

By introducing predator moth that feed on cactus

The correct answer is Option (3) - By introducing predator moth that feed on cactus

Predation is the interaction between prey and a predator, where the energy flows from one organism to the other. The predator is the organism that feeds on other organisms called prey.

Besides acting as ‘conduits’ for energy transfer across trophic levels, predators play other important roles. They keep prey populations under control. But for predators, prey species could achieve very high population densities and cause ecosystem instability. When certain exotic species are introduced into a geographical area, they become invasive and start spreading fast because the invaded land does not have its natural predators. The prickly pear cactus introduced into Australia in the early 1920’s caused havoc by spreading rapidly into millions of hectares of rangeland. Finally, the invasive cactus was brought under control only after a cactus-feeding predator (a moth) from its natural habitat was introduced into the country. Biological control methods adopted in agricultural pest control are based on the ability of the predator to regulate prey population.