Final answer:
The domain of a function is best described as the set of all possible input values for which the function is defined, which could be numerical or non-numerical.
Step-by-step explanation:
The description that best explains the domain of a function is d) Set of possible input values.
In mathematics, particularly in the study of functions, the domain refers to the set of all possible input values for which the function is defined. This may include numbers, like in real-valued functions, but can also involve other types of values, such as colors in the example of a random variable X representing hair colors with the domain being {black, blond, gray, green, orange}. It's crucial to identify the domain before performing experiments or analyses, as predictions outside this domain could be unreliable. The domain contrasts with the range, which represents the set of all possible output values that a function can produce, dependent on the given inputs.