The duration 19-21 days refers to:

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Q: 207 (SSC/NA)
The duration 19-21 days refers to:

question_subject: 

Science

question_exam: 

SSC

stats: 

0,7,8,3,3,7,2

keywords: 

{'incubation period': [1, 0, 0, 0], 'duration': [0, 0, 1, 0], 'menstruation': [1, 0, 0, 0], 'menstrual cycle': [1, 0, 0, 0], 'malarial parasite': [0, 0, 0, 2], 'period': [9, 3, 3, 8], 'red blood cell': [1, 0, 2, 2], 'fertilisation': [1, 0, 0, 0], 'days': [0, 0, 2, 0], 'schizogony cycle': [1, 0, 0, 0], 'average life': [1, 0, 0, 0]}

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.