Answer:
C. Mutation.
Explanation:
The mutation is the source of all genetic variation. There are two types of mutations. One is the natural mutation, and another is the artificial mutation. The natural mutation occurs due to the environmental cause, and humans using various mutagens do artificial mutations.
These two types of mutations always bring variation in any organism. This variation can be positive or can be negative too. Natural mutation is not possible to control so that anything can happen. Either positive or negative changes can occur. But in the case of artificial mutation, in 90% cases, positive results come in genetic variation.
