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

The correct answer is Option (1) → The host evolves mechanisms to reject or resist the parasite, and the parasite evolves countermeasures to overcome them.

Parasitism is an interaction in which the parasite benefits (+) and the host is harmed (–). Many parasites are host-specific, so host and parasite evolve together. This is co-evolution: each change in one is answered by a change in the other.

Option (1) The host evolves mechanisms to reject or resist the parasite, and the parasite evolves countermeasures to overcome them. — Correct. When the host evolves ways to reject or resist the parasite, the parasite must evolve ways to counteract and neutralise them. Only then can it go on living on the same host species.

Option (2) The host and parasite compete for resources and try to outcompete each other. — Incorrect. Two organisms fighting over the same resources is competition, not parasitism. In parasitism the parasite lives on or in the host and takes food from it.

Option (3) The host adapts to become a more suitable habitat for the parasite. — Incorrect. The host does not change to help the parasite. The host is harmed, so it evolves to resist the parasite, not to suit it better.

Option (4) The parasite evolves to become less harmful to the host. — Incorrect. Co-evolution here is an arms race between attack and defence. The book does not describe it as the parasite becoming less harmful.

NCERT Extract:
Population Interactions – “… (iii) Parasitism: Considering that the parasitic mode of life ensures free lodging and meals, it is not surprising that parasitism has evolved in so many taxonomic groups from plants to higher vertebrates. Many parasites have evolved to be host-specific (they can parasitise only a single species of host) in such a way that both host and the parasite tend to co-evolve; that is, if the host evolves special mechanisms for rejecting or resisting the parasite, the parasite has to evolve mechanisms to counteract and neutralise them, in order to be successful with the same host species.”