Choice C is correct. Sine and cosine are related by the equation:  Therefore,  which reduces to 

Choice A is not the correct answer. This answer may result from a misunderstanding about trigonometric relationships. A student may think that cosine is the opposite function of sine, and therefore think that the negative of the cosine of an angle is equivalent to the sine of that angle.

Choice B is not the correct answer. This answer may result from a misunderstanding of the unit circle and how it relates to trigonometric expressions. A student may think that, on a coordinate grid, the negative sign only changes the orientation of the triangle formed, not the value of the trigonometric expression.

Choice D is not the correct answer. The student mistakenly remembers the relationship between sine and cosine and adds  to the angle measure instead of subtracting the angle measure from