In probability theory, what is the probability of an event that is certain to occur?

• A. $0.5$
• B. $0$
• C. $2$
• D. $1$

Answer: (D)

Verified step by step guidance

1. Understand that in probability theory, the probability of an event is a number between 0 and 1 inclusive, where 0 means the event will never occur and 1 means the event is certain to occur.
2. Recall the definition of a certain event: it is an event that will happen every time the experiment is conducted.
3. Recognize that the probability of a certain event is always 1, because it is guaranteed to occur.
4. Note that probabilities like 0.5 represent events that have a 50% chance of occurring, and 0 or 2 are not valid probabilities for a certain event (2 is invalid since probabilities cannot exceed 1).
5. Therefore, the probability of an event that is certain to occur is expressed as \$1$.