I composed this before the "It really will be your greatest fear" thing got revealed but it's still relevant.
[X] Answer truthfully. You'll tell everyone what you saw.
-[X] You were told by someone whose approval you once looked for that you make everything actively worse just by existing.
It's a common fear at that age, hell, I'm nearly thirty and I still worry about that happening. Granted, at that age, I was having nightmares about my school administration hunting me through the streets on horseback.
If you'll excuse me for a second, I'm going to be incredibly sarcastic at the other options.
-[] You were attacked by your childhood protector who tried to kill you before you entered the Academy, and this time he was going to finish the job.
My worst fear is that a cunning fellow who I killed and everyone believes is dead has been secretly biding his time and is going to attack me in the middle of the village! Alternatively, some crazy ninja is going to seek immortality and bring back the dead to kill people he doesn't like! Good thing that neither of those things happens in canon to people who are far more important than us.
-[] You were in a world where nothing around you was real. A world where maybe you aren't even real.
My worst fear is the thing you just fucking did to me. Yeah, that's going to go over well. And couple that with her own past of having to deal with a reality warping Genjutsu user... Yeah, that's not going to look good at all, especially with our genius at the subject. Kurenai is one of the world's top genjutsu users who has the ability to semi-permantly lock away someone abilities, confessing that nothing feels real when we show evidence of an unknown bloodline? Not great.
Yes, being a Genjutsu genius in some ways makes this fear more likely, but it also points to us having something like the reality warper's bloodline jutsu. Not great. (Yes, she was anime only. She was also really terrifying on a conceptual level.)