The right answer is A mutation.
Alteration of the genetic material (DNA or RNA) of a cell or virus that causes a change in certain traits due to the inheritance of this genetic material from generation to generation.
Abnormal modification of the DNA of a gene, either spontaneously during cell division, or under the influence of external agents called mutagens. This gene is modified to be transmitted to the daughter cells. Some mutations have no effect on the cell. Others are the first step in a long process of cancerization.
Rare are the mutations that are beneficial.