I'm happy you enjoyed the update! For the choices, the option you're confused about is worded a little differently in the game. It says "Your chest was flat, but that might change as you got even older.". The word "might" is used rather than "probably". "Probably" implies that it likely will happen. While "might" just acknowledges that there's a theoretical chance that it could happen, but doesn't imply that it actually will.
If you pick that option, you'll have an MC with the potential to change but with no insistence any change will actually end up happening. So your options will be left open rather than being totally settled. When the MC gets older later in the game, that topic would come back up and you'd get to choose how it worked out.
Basically that choice option is good for MCs who might be late bloomers, or trans, or for players who just aren't totally sure what they wanna do with their character yet. Later on you can still say your character is fully flat chested, even if you picked the second option. You don't have to worry about having boobs unless you directly say your character has them, haha. But if you're positive you want a breast-less chest, the top option on the list would be the most effective option for your needs!
Sorry for intruding but can I ask you something? There'll always be the possibility to choose the MC's body aspect in the story? Like in the passage When you changed from kid to the next stage there was the possibility to choose you character's height (from very tall to very short). I really liked that aspect of the game, it made it more realistic!
Tall for a 13-year-old is pretty different from what's considered tall for an 18-year-old. It isn't too uncommon for someone to be the tallest kid in their class at that age, but totally stop growing from there and ultimately end up being shorter than most of their peers later in life. If you're talking about going from 18 to 23, the change between those ages will be handled differently.