31) C

The 4 stacks which are at the sides (not in the middle 6) have 6 cylinders. Since there are 20 cylinders on day 11, then there must be 14 cylinders in the middle 6 stacks.

We know that all 6 stacks must have at least one cylinder on each of them. The view from X shows 6 cylinders in the middle. There are 6 on the view from X and each of the stacks must have at least one cylinder. This means that a maximum of 4 cans can be missing.

A, B, E and F are too full. They definitely cannot the be the answers.

D is still possible if the middle 2 stacks only have one cylinder each. The middle six stacks would therefore have 14 cylinders.

C is not possible because the top middle two sacks would have to be 2 cylinders tall and middle two stacks would have to be 1 cylinder tall.

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