In order the answer this question you need to know what a synonym is:
Definition: A synonym is a word or phrase that has exactly the same, or nearly the same, meaning.
You must also know the meaning of mishap since that is the word you are asking to find a synonym for.
Definition: A mishap is defined as in an unfortunate accident.
Based on these definitions, you can see that your answer is: A. accident.
Hope this helps.
Take care,
Diana