In bacteria, variability is caused by

Answer: Option D

In bacteria, variability is caused by Conjugation, Transformation and Transduction.
Genetic exchanges among bacteria occur by several mechanisms.
In conjugation, the donor bacterium transfers DNA to the recipient by mating.
In transformation, the recipient bacterium takes up extracellular donor DNA.
In transduction, donor DNA packaged in a bacteriophage infects the recipient bacterium.