HIV's Genome of RNA Includes the Code for Reverse Transcriptase

HIV's genome of RNA includes the code for reverse transcriptase (RT) , an enzyme that acts early in infection to synthesize a DNA genome off of an RNA template.
The HIV genome also codes for protease (PR) , an enzyme that acts later in infection by cutting long viral polyproteins into smaller, functional proteins. Both RT and PR represent potential targets for antiretroviral drugs. Drugs called nucleoside analogs (NA) act against RT, whereas drugs called protease inhibitors (PI) act against PR.
Which mechanism produces variation for evolution by shuffling existing alleles?

A) rapid reproduction
B) sexual reproduction
C) mutation
D) changes in chromosome numbers