Assuming the ends of the range of estimates are +/-3 sigma from the mean, which of the following range estimates involves the LEAST RISK?

a. 30, days, plus or minus 5 days

b. 22-30 days

c. Optimistic=20 days, most likely=30 days, pessimistic=33 days

d. Mean of 28 days

Please let me know the right answer and explanation please of how to handle such questions.
I have no clue on this one.