Final answer:
Oogenesis is the biological process by which oogonia divide by mitosis into primary oocytes which then undergo meiosis to become secondary oocytes and eventually form an ovum upon fertilization. Therefore, the correct option is A.
Step-by-step explanation:
Oogenesis is the process of producing eggs in the ovaries of a female. This process starts during fetal development when a germ cell called an oogonium undergoes mitosis to produce multiple oocytes. At birth, a female has roughly one to two million oocytes. Each primary oocyte begins meiosis I and is then arrested in this stage until puberty. Once a female reaches reproductive maturity, these primary oocytes resume meiosis I during the ovarian cycle, resulting in the formation of a secondary oocyte. Upon ovulation, the secondary oocyte is released from the ovary, and completion of meiosis II occurs only if fertilization takes place, thus forming the ovum. Therefore, to answer the student's question: a) formation of an ovum is the correct definition of oogenesis.