Final answer:
Viruses play a key role in adaptive evolution by introducing genetic variation into host populations.
Step-by-step explanation:
The correct statement about adaptive evolution is statement B: Viruses play a key role in the process of adaptive evolution.
Viruses have the ability to introduce genetic variation into the hosts through viral replication and infection. This genetic variation can lead to adaptation and evolution in the host population.
For example, when a virus infects a population, some individuals may possess genes that provide resistance to the virus. These individuals are more likely to survive and reproduce, passing on the resistant genes to future generations. Over time, the resistant phenotype becomes more dominant in the population, illustrating the process of adaptive evolution.