Question

Probability of a sure event is _________

Answer 1

An event that tends to happen always is termed to be a sure event. As the name suggests, a sure event will happen no matter what.

For example, it's a sure event to obtain a head when tossing a coin since the possible outcomes of tossing a coin will be head or tail.

Therefore, the probability of a sure even will always be $1$ since the event occurs no matter what.