Final answer:
The domain of the function f(x), which is represented by a horizontal line, is all real numbers from 0 to 20, inclusive.
Step-by-step explanation:
To determine the domain of a function represented by a graph, we need to look at the range of x values for which the function is defined. For the given function f(x), which is a horizontal line, we see that it is defined from x = 0 to x = 20, inclusive. This means that every number between and including 0 and 20 is included in the domain of the function.
The function does not exist for x values outside of this range and therefore, the domain of f(x) is all real numbers such that 0 ≤ x ≤ 20.