A is wrong because the passage does not say or assume that the only purpose of keeping domestic cats is to have a positive effect on native wildlife.
B is wrong because the passage says, “domestic cats caught and killed so-called invasive species”. So it is certain that numbers of invasive species decreased because of the presence of domestic cats.
C is correct because the passage only gives the example of Canberra. The passage assumes that Canberra is representative of the general environmental effects of keeping domestic cats.
D discusses the purpose of keeping domestic cats. However there is nothing assumed about the purpose of keeping domestic cats in the passage.