Explanation:
What you would do is just find the surface area of each one, and the would would be your reasoning.
So to start with the 6 by 6 by 6 cube
The formula is
sa=2(wl+hl+hw)
So
Sa=2(6x6+6x6+6x6)
Sa=2(36+36+36)
Sa=2(108)
Sa=216
Next is the shape in the middle
You would use the same formula
So
Sa=2(4x4+13.5x4+13.5x4)
Sa=2(16+54+54)
Sa=2(124)
Sa=248
And for the last shape
Sa=2(1x16+13.5x1+13.5x16)
Sa=2(16+13.5+216)
Sa=2(245.5)
Sa=491
This making it that the cube has the lowest surface area.