The correct answer is C. (-∞, ∞).
The cosine function is defined for all real numbers as it takes an input of an angle (in radians) and outputs a value between -1 and 1. Therefore, the domain of y = cos(z) is all real numbers, which is represented as (-∞, ∞) in interval notation.
Option A is incorrect because [0, ∞) would be the range of the cosine function, not its domain.
Option B is incorrect because 2π is a period of the cosine function, not its domain.
Option D is incorrect because [-1, 1] is the range of the cosine function, not its domain.