Final answer:
The correct word to fill in the sentence is 'coda,' which signifies a concluding part of a series of events.
Step-by-step explanation:
December's earthquake was but a coda to a terrible year for a small island nation recently wracked by civil strife and devastating tropical storms. The term coda refers to a concluding event, characteristic, or section of a piece of music or literature. In this context, it signifies the last in a series of unfortunate events that happened over the year.
This choice makes sense given the information provided, as it suggests the earthquake was the final incident in a chain of catastrophic events. Other options, such as catharsis or homily, are not appropriate as they convey a sense of emotional release or a moralizing lecture, respectively.