Final answer:
The function value greatest at an input of 0 is f(0), which is always 20 within the interval [0, 20].
Step-by-step explanation:
The function value that is greatest when the input is 0 is f(0). Given the information provided, the graph of f(x) is a horizontal line, which means the function has a constant value for all x within the interval [0, 20]. Although additional information about the functions g(x), h(x), and k(x) are not provided, what we know about f(x) is that it is always 20 in the intended interval, hence f(0) = 20. Without the values of g(0), h(0), and k(0), we cannot definitively compare them, but f(0) has a very high constant value.