The maximum rainfall depth of 300 mm in 24 hours has a return period of 100 years. The probability of 24 hours rainfall equal to or greater than 300 mm occurring at least once in 10 years is given by A. 0.9910 B. 1 – 0.9910 C. 0.9100 D. 1 – 0.9100

The correct answer is: C. 0.9100

The probability of an event occurring at least once in a given period of time is equal to 1 minus the probability of the event not occurring at all in that period of time. In this case, the event is a 24-hour rainfall equal to or greater than 300 mm. The probability of this event not occurring in any given year is 0.0910, which is the reciprocal of the return period of 100 years. Therefore, the probability of the event occurring at least once in 10 years is 1 minus 0.0910, or 0.9100.

Option A is incorrect because it is the probability of the event occurring in any given year. Option B is incorrect because it is the probability of the event not occurring in any given year. Option D is incorrect because it is the probability of the event occurring at least once in 100 years.