Evolution involves natural selection working on a diverse gene pool and picking better adapted genotypes/phenotypes. In this case, the gene pool is the different kinds of virus, and host defenses serve as the agent of natural selection. The most diverse lineages/those least recogniwed by host defenses are the ones which survive.
Work Step by Step
To get this answer, go back and review basic principles about natural selection. See that the different varieties of virus in the host are like different species on which natural selection can act.