Kwin, the correct answer would appear to be 15 miles IF the question was stated without any mention of weather/visibility conditions.
The 9-mile range is indeed nominal range--and that only applies when the visibility conditions are ALSO defined as nominal, i.e. stated up front that they apply to the situation. If the visibility conditions are not mentioned in the question, then nine miles is wrong because it assumes something that is not so: Nominal conditions rather than optimal or maximum ones.
USSA has been known to make mistakes. Since you are taking their course, it might pay to raise this question with them and ask them to rephrase the question to remove the error or ambiguity.