To calculate the probability of either event A or event B occurring (denoted as P(A or B)), you use the formula:
\[ P(A \cup B) = P(A) + P(B) - P(A \cap B) \]
This equation takes into account both probabilities and subtracts the overlap (the probability that both A and B occur) since it would be counted twice when you add P(A) and P(B).
Therefore, the correct answer is:
C. \( P(A) + P(B) - P(A \text{ and } B) \)