a) The probability of an event that cannot occur is
Final Answer: The probability of an event that cannot occur is \( \boxed{0} \).
Step 1 :The probability of an event is defined as the number of favorable outcomes divided by the total number of outcomes.
Step 2 :If an event cannot occur, there are no favorable outcomes.
Step 3 :Therefore, the probability of an event that cannot occur is 0.
Step 4 :Final Answer: The probability of an event that cannot occur is \( \boxed{0} \).