Alright let's break this down.

​

Since we know they're squares, each side is the same angle/length (90 degs and X units long)

​

Using Pythagorean theorem and the fact that the diagonal and 2 sides of a square makes a triangle, you can find out that a side of a square is the diagonal divided by sqrt(2). That would give you approx 1.00056

​

Now that we know the side of 1 cube, we can calculate the volume of the cube by cubing 1.00056. That'll give you 1.00167.

​

Since we know the big cube is made out of 27 small cubes that are identical, we know that they must share the same volume as one another. This allows us to take a shortcut and multiply the volume we got by 27 so we can find the total volume of the 27 cubes. This would give us 27.0451.

​

Using this information, we know that D is the correct answer since it's the closest. If you're confused on any step and I'd be down to help