Rewrite the Expression with a Positive Rational Exponent $9 ^ { - 1 / 2 }$

Multiple Choice

Rewrite the expression with a positive rational exponent. Simplify, if possible.
- $9 ^ { - 1 / 2 }$

A) $- \frac { 1 } { 3 }$
B) $- 3$
C) $\frac { 1 } { 9 }$
D) $\frac { 1 } { 3 }$