The answer depends on what you're actually asking:
1. The quiz includes Washington as a valid answer because the name of the city is just "Washington" which ends in N. The "D.C." part is not part of the city name, but is just included by most Americans to distinguish it from e.g.: Washington state.
2. The quiz instructions mention it because to some people it's not obvious whether it should be included or not. Sometimes quiz setters forget that the "D.C." isn't part of the name and leave it out. Could the instructions have skipped it and left it to quiz takers to try it and see for themselves? I suppose, but let's be honest. It's only one answer out of 24; it's not really that big a deal to be explicit up front.
1. The quiz includes Washington as a valid answer because the name of the city is just "Washington" which ends in N. The "D.C." part is not part of the city name, but is just included by most Americans to distinguish it from e.g.: Washington state.
2. The quiz instructions mention it because to some people it's not obvious whether it should be included or not. Sometimes quiz setters forget that the "D.C." isn't part of the name and leave it out. Could the instructions have skipped it and left it to quiz takers to try it and see for themselves? I suppose, but let's be honest. It's only one answer out of 24; it's not really that big a deal to be explicit up front.