The correct answer is: A. Atomic event
An atomic event is a single, indivisible event that cannot be broken down into smaller events. It is the complete specification of the state of the world at a given point in time.
A complex event is a collection of atomic events that occur together. It is not a complete specification of the state of the world, as it does not specify the order in which the atomic events occurred.
A simple event is an atomic event that does not have any sub-events. It is a complete specification of the state of the world at a given point in time, but it is not a complex event.
None of the mentioned is not a valid answer.