You have stated the rule correctly. 2 must be at least one shelf above 6. However, I don't think you are visualizing the shelves properly.
If I arrange it from top to bottom, this is what answer choice C looks like.
T: 1 3 5 M: 4 6 B: 2
As you can see, dish 2 is below dish 6 in this scenario, so we can eliminate answer choice C. Note that dish 2 can never be on the bottom shelf, because it has to be somewhere above 6.
