Find the Exact Length of the Diagonal (In Blue) of One

Multiple Choice

Find the exact length of the diagonal (in blue) of one of one of the faces of the cube.

A) 10 cm
B) 15 cm
C) $2 \sqrt { 10 }$ cm
D) $5 \sqrt { 2 }$ cm
E) $\frac { 5 \sqrt { 2 } } { 2 }$ cm