I think C
R + B + G = Total
We want to find G (key point to remember is G < 15 and B, G, and R have to be a positive integers)
(1) R / B = 6 / 5
This along isn't sufficient as we cannot figure out the total OR the G. For example, total can be 12 with 6 R, 5 B, and 1 G. Alternatively, total can be 13 with 6 R, 5 B, and 2 G. And any number of other solutions.
NOT SUFFICIENT.(2) G = 2R
This still doesn't give us a unique solution of G. Again, we can apply virtually infinite number of different cases.
NOT SUFFICIENT.(1) + (2)
We can sub in R = (1/2)*G from (2) into the ratio from (1)
(1/2) * G / B = 6 / 5
G = (6 * 2 * B) / 5
From this, we find that G has to be a multiple of 5 AND < 15, implying that B has to be a multiple of 5 (as 6 and 2 are not multiples of 5)
The only solution that satisfies this B = 1, G = (6 * 2 * 5) / 5 = 12
Going to B = 10 (next multiple of 5) -> G = (6 * 2 * 10) / 5 = 24, which is not possible
Hence, there is a unique solution so the combination of statements is
SUFFICIENTC