The process described is C. mitosis.
In mitosis, a single cell divides to produce two daughter cells that are genetically identical to the original cell, maintaining the same chromosome number. Since the original cell had four chromosomes and each daughter cell also has four chromosomes, this indicates that mitosis has occurred.
Meiosis, on the other hand, results in four daughter cells (not two) and reduces the chromosome number by half, which is not consistent with the scenario described. Fertilization is the fusion of gametes and does not directly involve the division of a cell.