I wanna say that this is the chapter that she fell in love with him. Here's my thought process, chapter 11 they're still just friends. She finds it a bit weird but is perfectly fine taking a picture with him and doesn't really blush whatsoever. That means that the time she fell in love had to be some time after 11. Meanwhile she is definitely in love with him by chapter 16 judging by how she acts (wanting to join his club, riding the same bike, gifting him food etc), That means it'd have to be some time between chapter 11 and 16. chapter 13 nothing particularly important happens, 14 is where he realizes his feeling for her but he doesn't do anything in particular that she'd know about for her to fall for him then, which leaves chapter 15 this one, and 12, and although she does blush a small bit during chapter 12 there's no particular "event" per say that would have her falling in love with him, hence why I believe this to be the chapter she fell for him. As for why I did all this I was curious to figure out where it all started for her since we already knew the chapter the guy fell in love with yamada.