The process of human spermatogenesis produces four sperm cells. Of four cells produced, two of m have 20 chromosomes, and two of m have 26 chromosomes. The non-disjunction that caused cells to have an unbalanced chromosome number probably occurred during which stage of meiosis?  

Human sperm cells have 24 chromosomes. During the formation of sperms, the diploid primary spermatocytes having 46 chromosomes undergo meiosis to produce haploid sperms. In meiosis I, there is reduction of chromosome number to half whereas, meiosis II is an equational division as chromosome number remains the same. If the sperm cell has 26 chromosomes instead of 24, then it means it has 2 extra chromosomes. This occurred due to non disjunction of two chromosome pairs which failed to segregate during Anaphase I of meiosis I. As a result, one cell received two extra chromosomes and one cell received two chromosomes less. 

So, the correct answer is 'Meiosis I'.