This question asks for the "most likely" reason for why the individual chromosomes present in a spermatozoan at the end of meiosis are different than the individual chromosomes that were present at the start of meiosis. The most likely reason for the genetic changes is due to a process called "crossing over." Crossing over occurs when sections of homologous chromosomes exchange places with each other. This happens at the start of meiosis when the homologous chromosomes pair up together so closely that random sections of the genetic code move from one chromosome to another. The result of crossing over is that individual chromosomes now contain sections of paternal genetic information along with sections of maternal genetic coding; therefore, the individual chromosomes that exist at the end of meiosis are unique and different from the chromosomes that were used at the start of the process.
Crossing over helps to create even more genetic diversity within a sexually reproducing species. Combining the DNA from a mom and dad during fertilization does create genetic diversity; however, crossing over enhances the genetic diversity because it ensures that the genetic information in any given sperm cell or egg cell is likely unique genetic information since the crossing over occurred randomly and before the reproductive cells were completed.