Find the Exact Value of the Expression, If Possible $\cos ^ { - 1 } \left[ \cos \left( - \frac { \pi } { 6 } \right] \right.$

Multiple Choice

Find the exact value of the expression, if possible. Do not use a calculator.
- $\cos ^ { - 1 } \left[ \cos \left( - \frac { \pi } { 6 } \right] \right.$

A) $\frac { \pi } { 6 }$
B) $\frac { 5 \pi } { 6 }$
C) $- \frac { \pi } { 6 }$
D) $\frac { 7 \pi } { 6 }$