We have been given that probability of the even is 100%. In other words we can say that the probability is 1. It means that the event will certainly occur.
Now, we know that the highest value of probability is 1 and lower value is 0.
Mathematically, we can represents it as
If the probability is 0 then the event will never occur.
And if the probability is 1 then event will definitely occur.
Therefore, A certain event has a 100% chance of occurring.