So all the young or yet to be born people who will be future film fans don't deserve to watch an old movie unspoiled? Its just showing a bit of consideration.
If you are actively seeking out and doing quizzes, I'm guessing that such a common trivia question such as this has already crossed your path somewhere. I knew about it LONG before I attempted to watch Citizen Kane.
ECon211 - Charles Schulz joked about this not really being a spoiler as far back as 1973 and as recently as 1998. Tiny Tooons did a whole episode spoofing it in 1990. Family Guy joked about it in 2001. The Silver Surfer revealed it in 2014. The trope regarding spoilers that aren't spoilers anymore is called "It Was His Sled." The spoiler ship on this has sailed.
@MarlowePi: about 17-18 years ago I had a Mickey Mouse comic book. There was a story in it with Donald Duck and his nephews where Donald was really sad because of his sled "Rosetta" which was lost in his childhood. Only after this quiz I realized that the comic was a reference to the movie. It's never too late, I guess.
The 'mystery' of rosebud is a complete red herring. The real significance of the word lies outside the film, as it was Welles reference to the private life of the real newspaper owner on whom he modelled Kane.
Throughout the whole film, the meaning of the word 'Rosebud' is at the forefront of the viewers mind. So the reveal at the end is a very revelatory experience that strengthens all of the subtextual ideas that Welles was putting forth. Whatever Welles was commenting on outside of the film does not diminish the importance of the content within the film itself.
I first saw Citizen Kane on tv when I was ten or so. I immediately guessed within the first few minutes of the movie that "Rosebud" was the name of his sled. I might not have guessed correctly if I had first seen it as a thirty-year-old, but it was obvious to the ten-year-old me, who treasured the Fearless Flyer sled I used to coast down hills when it snowed.
I think spoilers should be mandatory for such old films. Otherwise how would younger people know they're worth watching if they don't know anything about them?
Speaking of which, the Titanic sinks.
(Especially when there are two questions here that do that!)