Answer D.
Using Pythagorean theorem
a^2 + b^2 = c^2
C^2 (8) is the long diagonal, it is the hypotenuse and (6) is one leg.
6^2 + b^2 = 8^2
rearrange to isolate the unknown
b^2 = 8^2 - 6^2
B^2 = 28
Take square root of both sides
b= 5.3, same as (2 sq.rt. 7) answer D
