Which of the following is not equal to cos⁡(520°)?
cos⁡(160°)
cos⁡(200°)
cos⁡(20°)
-cos⁡(-20°)

explain your answer

The cosine repeats itself every 360 degrees, and it mirrors at 0 and 180 degrees. It inverts around 90 and 270.
So without using a calculator, you can tell that:

cos(520)=cos(520-360)=cos(160)
cos(160) = cos(180-20) = cos(180+20) = cos(200)
cos(160) is NOT equal to cos(20), it would be -cos(20).
cos(160) = -cos(20) = -cos(-20)

So C is the only one unequal.