The fractions represent the ratios blue/white: 5/12, 6/6, 4/12, 5/6.
To make these fractions comparable multiply by 2 the numerators and denominators of the fractions with 6 as the denominator: 6/6=12/12 and 5/6=10/12. We now have four easy numbers: 5, 12, 4, 10. 4 is the smallest so the lowest ratio is 4/12 (=1/3), answer C.