This is a good question on which to start with a made-up number. Recognizing that we'll have to divide by 2 and by 3, let's say the capacity of C is 30. If it's half full of water, then, it contains 15 of water (there's no unit, but we don't need one). D has twice the capacity, so its capacity is 60. It's 1/3 full of water, so it contains 20.

The next step has us pouring as much of what's in D as possible into C. C has a capacity of 30 and is currently holding 15, so it could hold 15 more. D currently holds 20, so 15 of that could be poured into C, leaving 5.

If D has a capacity of 60 and is currently holding 5, it contains 5/60 of it's capacity, or 1/12, choice (C).