Which of following can be called as a mutation?

Meiosis I includes pairing of homologous chromosomes during prophase I and their segregation to opposite poles during anaphase I, this ensures that each gamete receives half the number of chromosomes as in somatic cell. Random fertilization of gametes during syngamy restores the somatic cell chromosome number in the zygote. Thus, reduction of chromosome number to half maintain the somatic chromosome number in the zygote which would otherwise multiply exponentially during each reproductive cycle. Reduction and syngamy are not mutations as there is no change in chemical structure of genes at a molecular level; options A and B are incorrect. The presence of additional chromosome, depending on a number of extra chromosomes, results in euploidy or aneuploidy. It is the chromosomal aberration, not the mutation; since, chromosomal aberrations and point mutations produce same phenotypic effects, the former is sometimes referred to as chromosomal mutations. Thus, the correct answer is option C.