The correct answer is option 3, which states that the duration 19-21 days refers to the period of the menstrual cycle soon after menstruation, during which fertilization is most likely to occur.

Option 1, "incubation period of a hen`s egg," is not the correct answer as the incubation period of a hen`s egg is typically around 21 days, not 19-21 days.

Option 2, "average life-span of a red blood cell in humans," is also not the correct answer as the average life-span of a red blood cell is approximately 120 days, not 19-21 days.

Option 4, "period of completion of one schizogony cycle of malarial parasite inside a red blood cell," is unrelated to the duration of 19-21 days mentioned in the question.

Therefore, the correct answer is option 3, the period of the menstrual cycle soon after menstruation, during which fertilization is most likely to occur.