B.31 inches, I got this from 30.88 and I rounded.
1st. Draw a line diagonal from one top corner to the opposite bottom corner(you will have 2 triangles)
2nd.Pythagorean theorem to figure out the hypotenuse of the triangle.
c^2=15^2 + 27^2 c^2= 225+729 c^2= 954 c= square root of 954 c=30.88, aka 31 inches.