Since the number on the faces of the cube are
1, 2, 3, 4, 5, 6
Then the total outcomes = 6 numbers
We need the multiples of 3
There are only 3 and 6 are the multiplies of 3, then
The outcomes = 2 numbers
The rule of probability is

n (x) = 2
Tn = 6

Simplify it by dividing up and down by 2

The probability of choosing a multiple of 3 is 1/3
The answer is B