Evaluate Without Using a Calculator by Using Ratios in a Reference

Multiple Choice

Evaluate without using a calculator by using ratios in a reference triangle.
- $\cos 210 ^ { \circ }$

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