The question asks us which color combos are possible after mixing all four of the initial flasks, which is two experiments.
For this kind of could be true, I would start by knocking out the answers that look facially impossible, like A and E, since there's only one way to get orange, and either of the ways for blue use up all of three.
Therefore we only need to actually look at three remaining possibilities.
For B: orange uses up two and three, and since three is needed to make blue, this is impossible.
For C: red requires 1 and 2, and we can make blue from 3 and 4, so this is possible.
For D: red requires 1 and 2, and green requires one of those two, so it is impossible.
