(sin²(x) - 1)/cos(x) = -1
-cos²(x)/cos(x) = -1
-cos²(x) = -cos(x)
cos²(x) - cos(x) = 0
cos(x) (cos(x) - 1) = 0
Then it follows that cos(x) = 0 or cos(x) = 1. But if cos(x) = 0, then the original expression on the left side of the equation would be undefined. Then C is the only valid solution.
