Correct option is A
From the given options
Accident: A human-induced or unplanned event.
Flood: A natural disaster.
Earthquake: A natural disaster.
Volcano: A natural disaster.
Analysis:
Flood, Earthquake, and Volcano are natural disasters caused by environmental phenomena.
Accident, however, is generally human-induced and not a natural disaster.
Odd One Out: A. Accident
Correct answer is (a).