The correct option is C 1
A certain event means an event which is sure to happen.
The probability of a certain event is 1.
For example, "Getting a number less than 7 when a fair dice is rolled" is a certain event.
A dice has 1,2,3,4,5 and 6 printed on it. So, we are sure that whatever comes will be less than 7. So, we thus conclude that the probability of a certain event is 1