Final answer:
The domain of the function y = cos(x) is all real numbers, which is option c. (-∞, ∞).
Step-by-step explanation:
The question asks what the domain of the function y = cos(x) is, where x represents the independent variable and y the dependent variable. In mathematics, the domain of a function is the set of all possible input values (x-values) for which the function is defined. The cosine function is defined for all real numbers because it represents the ratio of the adjacent side to the hypotenuse in a right-angled triangle, or more generally, the cosine of an angle in a unit circle, for any angle value. Thus, it can take any real number as its argument. Therefore, the correct answer is c. (-∞, ∞), which means that the domain of the cosine function is all real numbers.