Final answer:
The domain of g(f(x)) is the set of all values of x for which the composite function is defined, typically corresponding to the interval where f(x) is defined and g accepts all the values in the range of f.
Step-by-step explanation:
The domain of g(f(x)) refers to the set of all input values x for which the composite function is defined. Since f(x) is represented as a horizontal line between 0 ≤ x ≤ 20, the values of f(x) are defined for all real numbers within this interval.
Consequently, g(f(x)) will also be defined for these values if the function g accepts all the values in the range of f. In most cases, if g does not impose any additional restrictions, the correct response would be C) Values that make the composition defined. Without knowing the specific function g, this answer is based on the given range of f(x), and one would typically use a calculator to check for any restrictions imposed by g within this range.