Final answer:
An egg cell matures and passes through the fallopian tubes about every 28 days, which corresponds to the typical menstrual cycle length. Ovulation occurs around day 14, which is the primary opportunity for fertilization if sperm are present.
Step-by-step explanation:
One egg cell matures and passes through the fallopian tubes about every 28 days. During the menstrual cycle, ovulation occurs around the midpoint, effectively on day 14 in a typical 28-day cycle. This is when the mature follicle bursts open, releasing the secondary oocyte, which is then swept into the nearby fallopian tube. Ovulation is the crucial time when fertilization can occur if sperm are present. Once the egg is released, the follicle transforms into the corpus luteum.
If fertilization does not take place, the secondary oocyte will not complete meiosis II and will disintegrate. On the other hand, if the oocyte is fertilized by a sperm, it quickly completes meiosis II, forming a diploid zygote, which travels through the fallopian tube and eventually implants in the uterus. Hence, the correct answer to the question is option a. every 28 days.