Sakura might be completely shocked at first. She might freeze for a moment, not believing what's happening. Then she could start crying tears of joy and say yes immediately.
Sakura might also be really flustered. She might blush deeply and stutter out her answer. She has feelings for Naruto (in this fanfiction scenario), but the suddenness of the proposal would still catch her off guard. She would then gather herself and say yes, and they would share a big hug.
She could be hesitant at first. Sakura has always been focused on her own growth as a ninja, and marriage is a big step. But after Naruto explains how he wants them to grow together and support each other, she would warm up to the idea and accept the proposal with a big smile.
Sakura would be over the moon. She would blush deeply and look at Sasuke with so much love in her eyes. She might be at a loss for words for a moment, but then she would find her voice and tell Sasuke that she has been waiting for this moment her whole life. She would also feel a sense of pride that Sasuke has chosen her, after all they have been through. And she would immediately start thinking about how to share the good news with their friends, especially Naruto, who has been like a brother to her and has also been a part of their journey.
She might be heartbroken. Sakura could initially be in shock and then fall into a deep sadness. She may blame herself for Naruto's leaving, thinking that she could have done something differently.
One possibility is that Naruto finally gathers the courage to propose to Sakura in a very public and grand way. Maybe he takes her to the top of Hokage Rock, where they have a beautiful view of the village. He gives a heartfelt speech about how she has been by his side through so much, and how he can't imagine his future without her. Then he presents a ring that he's had made especially for her.
She could be heartbroken. Sakura has always loved Sasuke deeply, so his leaving for Naruto would be a huge blow to her. She might initially be in denial, not believing that Sasuke would choose Naruto over her.
She might be really hurt and start to question her self - worth. Maybe she becomes more withdrawn and less confident in herself.
Sakura may react with a sense of urgency. She would drop whatever she was doing and rush to Naruto's side. She'd be worried that something truly terrible has happened. If Naruto is hesitant to talk, Sakura might just sit quietly with him, waiting for him to open up. This would show her patience and understanding, and also how much she values Naruto as a friend.
Sakura would be devastated. She would probably blame herself for Naruto's death since he died protecting her. She might go into a state of deep depression at first, withdrawing from everyone.
At first, Naruto might be really confused. He's used to the normal Sakura, so when she starts acting all yandere, he'd be like 'What's going on, Sakura - chan?'. He'd probably try to talk to her and find out what's wrong.
She might be confused at first. Sakura is used to Naruto being friendly and now his coldness would be a big change. She may start to question what she did wrong.
She might be confused at first. Sakura is used to Naruto being friendly and always there for her, so his coldness would be unexpected. She may start to question what she has done wrong.