Final answer:
The probability rule used to calculate the occurrence of one or another event is the Addition Rule, which adds the individual probabilities of each event and subtracts the probability of both events occurring together. Option number a is correct.
Step-by-step explanation:
The probability rule that will determine if one or another event will occur is known as the Addition Rule. This rule is applied when calculating the probability of either event A or B happening, but not both simultaneously.
According to the Addition Rule, the probability of A or B occurring is expressed as P(A OR B) = P(A) + P(B) − P(A AND B), where P(A) is the probability of event A occurring, P(B) is the probability of event B occurring, and P(A AND B) is the probability of both events A and B occurring together. This rule is especially important when dealing with mutually exclusive events or when incorporating more than one pathway for an event to occur.