Find the Exact Value of the Expression $\cos 840 ^ { \circ }$

Multiple Choice

Find the exact value of the expression.
- $\cos 840 ^ { \circ }$

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