Final answer:
Cosine of 120° is equivalent to the negative cosine of its reference angle 60°, so the expression equivalent to cos 120° is -cos 60°.
Step-by-step explanation:
The expression equivalent to cos 120° can be found using the concept of reference angles and the cosine function's even-odd properties. Since 120° is in the second quadrant, where cosine values are negative and its reference angle is 60°, cos 120° is equivalent to -cos 60°. Therefore, the correct expression equivalent to cos 120° is option (a) -cos 60°.