Final answer:
An event is considered 'probable' if it has a high likelihood of occurrence, frequently denoted by a probability value close to 1; in this context it is defined as a common outcome.
Step-by-step explanation:
In probability theory, an event is considered probable if it has a high chance of occurring. The theoretical probability of an event A, when all outcomes in the sample space are equally likely, is calculated by counting the number of outcomes for event A and dividing by the total number of outcomes in the sample space. For instance, if the probability of an event is equal to .498, it is very close to .5, indicating that the event is equally as likely as not to occur, which can also be considered a common outcome in many situations.
However, the option that defines an event as 'probable' in the terms of the question provided would be b) small enough to qualify as a common outcome, suggesting that the event occurs frequently enough to be considered 'probable'.