Final answer:
The rare disorder described would most likely be caused by a mutation since the parents have only normal copies of the gene, indicating it is not inherited but due to a spontaneous genetic change.
Step-by-step explanation:
The rare disorder being described in the question is most likely caused by a mutation (C). Parental genes that are normal indicate that the disorder is not inherited but has occurred due to a spontaneous change in the gene's sequence. Mutations can arise spontaneously from errors during DNA replication, or they can result from environmental factors such as radiation or exposure to toxic substances.
While gene splicing might be a method used in laboratory settings to manipulate genes, it is not a natural process that would cause a genetic disorder. Mitosis is a process of cell division that produces two identical cells and would not by itself cause a genetic disorder unless a mutation were to occur during the process. Natural selection is an evolutionary process that can lead to the prevalence of certain traits within a population over time, but it is not an immediate cause of a genetic disorder.
Some genetic disorders can be caused by the presence of an incorrect number of chromosomes, like Down syndrome, which is due to trisomy 21 and arises from chromosomal nondisjunction during meiosis. However, since the parents have only normal copies of the gene in question, the disorder in this case is not chromosomal but rather due to a mutation within a single gene.