To determine which of the options is least likely to occur, first convert all of them to the same form (fractions, decimals, percentages, etc.). Since 2 are already in fraction form, let's convert the other one to fraction form as well.
0.4 = 4/10
4/10 = 2/5
Now, the probabilities are 2/5, 1/6, and 2/3. Since 1/6 is the smallest number out of the three, that option is correct; a probability of 2/3 the least likely to occur.