a) The probability of an event that cannot occur is

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 \( \boxed{0} \).