Evaluate the Expression Without Using a Calculator $4 ^ { - 3 / 2 }$

Evaluate the expression without using a calculator. $4 ^ { - 3 / 2 }$

A) -8
B) 4
C) -4
D) $\frac { 1 } { 8 }$
E) $- \frac { 1 } { 8 }$