Correct answer: C). Frequent Mutations
A virus is a small infection-causing agent that are known to replicate inside the living organism. They may contain DNA or RNA as genetic material.
They evolve by the changes which caused in their RNA sequence, as they carry RNA as a genetic material which is not stable so they continuously change their shape as a result of frequent mutations in order to infect an organism and escape from the immune system of the body of the host.
Hence, the correct answer would be option C.