In parasitism, the co-evolution of host and parasite occurs when:

The host evolves mechanisms to reject or resist the parasite, and the parasite evolves countermeasures to overcome them.

The correct answer is (a) The host evolves mechanisms to reject or resist the parasite, and the parasite evolves countermeasures to overcome them.

In parasitism, the host and parasite have a close relationship where the parasite benefits at the expense of the host. As this interaction continues over time, both the host and parasite can undergo co-evolution, which means their evolutionary paths become intertwined. This co-evolutionary process occurs when the host evolves mechanisms to defend itself against the parasite, such as immune responses or physical barriers, while the parasite evolves countermeasures to overcome these defenses and continue exploiting the host.

This ongoing cycle of adaptation and counter adaptation is driven by natural selection. Hosts that can better resist or reject parasites have a higher chance of survival and reproduction, leading to the selection of traits that enhance their resistance. On the other hand, parasites that can evade or overcome host defenses have a higher chance of successfully infecting and reproducing within hosts, leading to the selection of traits that enhance their ability to infect.

Therefore, the co-evolution of host and parasite occurs as a result of the continuous pressure imposed by each species on the other, leading to an ongoing arms race of adaptations and counter adaptations.