Which formula gives the x -coordinates of the maximum values for y is equal to cosine of x ?

The cosine function reaches its maximum value of 1 at x = 2πn, where n is any integer. Choice C gives x = kπ with k an even integer, so its values are ..., −4π, −2π, 0, 2π, 4π, ... . These are exactly the x-coordinates of the maxima.

AI-written and checked against the verified answer.