Final answer:
The midline of the function f(x) = 2(cos(x - 5π/6)) is y = 0, so the correct answer is d) x = 0, as midline refers to a y-value.
Step-by-step explanation:
The midline of a trigonometric function like
represents the horizontal shift of the cosine function. Considering that the standard cosine function, cos(x), has a midline at y = 0, any vertical shift would alter this midline. However, since this function does not have a vertical shift (no constant is added to the function), the midline of the transformed function remains unchanged at y = 0. Therefore, the correct answer is d) x = 0, as midline refers to a y-value, not an x-value, and in this case, it is the y-value where the function oscillates around.