Without Using a Calculator, Give the Exact Trigonometric Function Value $\frac { 1 } { 2 }$

Multiple Choice

Without using a calculator, give the exact trigonometric function value with rational denominator.
-cos 30°

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