Which of the following numbers represent the probability of an event that is less likely to occur than not occur?
1/5
1/2
0.25
0.75

Well,

The probability that is least likely to occur is the smallest probability.

0.2
0.5
0.25
0.75

0.2, or 1/5, is the smallest number and therefore the probability which is the least likely to occur.

Converting them all to decimals you end up with:
0.2
0.5
0.25
0.75

so it would be 1/5, if I understand the question right