Flower color in primrose plants is controlled by an individual gene. the sudden appearance of one white flowering primrose in a plant breeder's field of red primrose plants is most likely due to

Flower color in primrose plants is controlled by an individual gene. The sudden appearance of one white flowering primrose in a plant breeder's field of red primrose plants is most likely due to mutation.
A mutation represents the permanent alteration of the nucleotide sequence of the genome of an organism in this case DNA of the plant.  Mutation can potentially lead to the inactivation of some enzyme involved in the biosynthetic pathway of the pigment.