In the rhythm method of contraception, the first day of the fertile period is determined by –

Correct option is C

Sol. In the rhythm method of contraception, the fertile period is determined by subtracting 18 days from the shortest menstrual cycle to find the first fertile day. The last fertile day is calculated by subtracting 10 days from the longest cycle. This method helps estimate the ovulation window for natural family planning.
Explanation of Options:
· (a) Longest cycle - 20 days: Incorrect; longest cycle is used for the last fertile day, not the first.
· (b) Longest cycle - 12 days: Incorrect; does not accurately determine the first fertile day.
· (c) Shortest cycle - 21 days: Correct answer; the first day of the fertile window is determined using the shortest cycle minus 18.
· (d) Shortest cycle - 11 days: Incorrect; does not reflect the standard calculation method.