Find the Exact Value of the Expression $\cos \left[ \tan ^ { - 1 } \left( \frac { \sqrt { 3 } } { 3 } \right) \right]$

Multiple Choice

Find the exact value of the expression.
- $\cos \left[ \tan ^ { - 1 } \left( \frac { \sqrt { 3 } } { 3 } \right) \right]$

A) $\frac { \pi } { 3 }$
B) $\frac { 1 } { 2 }$
C) $\frac { \sqrt { 3 } } { 3 }$
D) $\frac { \sqrt { 3 } } { 2 }$