The prop ability that an event will occur is 1. Which of the following best describes the likehood of the event occurring?

When the probability of an event is = 1 that means it's no longer a probability but a certainty, an assertion instead.

If today is Thursday, what is the probability tat tomorrow will be Friday?

It's equal to 1 since certainly tomorrow will be Friday

However In noticed that your problem is incomplete, hence we can't answer your last question