The correct answer to this question is:

a. union

Explanation:

In probability and set theory, the union of two events $A$ and $B$, denoted as $A \cup B$, represents all instances in which either event $A$, event $B$, or both events occur. This matches the definition provided in the question.

Would you like further details on the concept?

1. What is the mathematical representation of the union of two events?
2. How does the union of events differ from the intersection of events?
3. Can an event have both union and intersection? Explain.
4. How is the union operation used in probability calculations?
5. What is the complement of an event in probability?

Tip: In set notation, the union of two sets $A$ and $B$ includes all elements that are in $A$, $B$, or both.