Comparing the stability of geminal diols
Your reasoning is correct, but I think you'd mistakenly compared the ring strain of the rings, while I would have compared the ring strain in the corresponding ketones.
Note that, you are asked about the stability of gem-diols, which could convert into the corresponding ketone. The point is, the ring isn't going to change, so the stability of rings don't matter.
Now, as you know the corresponding cyclic ketones would be $\ce{sp^2}$ hybridized. So, the ring angle should be $120^\circ$ in order to minimize the ring strain in ketone. The most closest is in C and least is in A.
Hence, the order.