Wow. I see my mistake, at least in that the cube would have a higher length because the coating applies to every face a 0.2 inch deep surface. Also, after considering my volumetric cube minus cube more approach appropriately applied I found that it was much greater than any answer.

That, along with your comment about extrusion lead me to the answer A with much greater surety.

As for the algebra, I'm going to say it's A upon further consideration. I'm still not quite sure, but that's all the factors of both multiplied together. Maybe that's not what I need here though.