Answer: 2/3
Reason:
Event space = {1,3,4,5} = set of outcomes we want to happen
Sample space = {1,2,3,4,5,6} = set of all possible outcomes
There are 4 items in the event space out of 6 items in the sample space. The probability we want is 4/6 = 2/3
Side note: The event space is a subset of the sample space.