Final answer:
Based on the information given, none of the provided options match the context of f(x) being a horizontal line and positive over the interval [0,20]. Hence, no interval from the options given can be accurately picked, and we decline to conjecture outside the given data.
Step-by-step explanation:
The question asks to find the intervals of x where f(x)>0. From the provided options, we must evaluate which interval correctly represents the x values for which the function takes on positive values. None of the information about probability distributions, uniform distributions, or continuous probability density functions provided directly informs us about the nature of f(x) besides its domain restriction between 0 and 20.
Additionally, because it's stated that f(x) is a horizontal line, if we consider f(x) to be a constant value that is positive from 0 and 20, the function will then indeed be greater than zero within the interval [0, 20]. However, the options given in the question do not correspond to the information provided about the function f(x). Therefore, instead of guessing amongst the unrelated options, we courteously decline to provide an incorrect choice and highlight the mismatch between the question and the provided context.