Why Are Such Phenomena as Hurricane Katrina and Rare Earthquakes

Why are such phenomena as Hurricane Katrina and rare earthquakes in regions where oil shale is fracked called "hundred-year" events?

A) They take place every other century.
B) They can be statistically averaged as taking place every 100 years.
C) They are statistical outliers that typically do not occur within a normal human lifespan.
D) That is a folkloric term intended to be hyperbole.