The correct option is D Sperm
Spermatogenesis is the process of formation of the sperms (the male gametes). It takes place in the seminiferous tubules of the testis. Germinal epithelium which lines the seminiferous tubules proliferate and then undergoes mitosis and meiosis, therefore from one primary spermatocyte four sperms are produced. This process usually takes up around 72-74 days. Spermatogenesis takes place throughout the life cycle of the males. So, the correct option is 'Sperm'.