If Naruto sings to Sakura during a time when she's vulnerable, like after she's been injured on a mission, she might be very moved. She could start to cry, because Naruto's song would be a sign of his care and support. And this might make her re - evaluate her feelings for Naruto in a more positive light.
Sakura could be initially embarrassed, especially if Naruto sings in a very public setting. But as she listens to the lyrics, she might start to feel a warmth in her heart. If Naruto is singing about his love for her, she might be conflicted between her old feelings for Sasuke and this new, unexpected display of affection from Naruto. She could end up blushing and not really knowing how to respond right away.
She might be surprised at first. After all, Naruto isn't known for his singing usually. But then she could be touched if she realizes the song is for her and about their relationship, whether it's friendship or something more.
She would be heartbroken. Sakura has known Naruto for a long time, and his death would shatter her world. She may become more withdrawn and serious. In the long run, she could use her pain as motivation to become stronger. Maybe she would train harder than ever before to make sure that no one else has to suffer the way Naruto did. She might also start to see Naruto in a different light, realizing all the things she took for granted when he was alive.
She would also be deeply emotional. Sakura has deep feelings for Naruto, even if they were not always fully realized or expressed. His sacrifice would make her confront these feelings head - on. She could be seen crying often, visiting Naruto's memorial frequently, and having long internal monologues about their relationship and what his sacrifice means to her.
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.
She might be initially shocked and hurt. Sakura has always had Naruto as a constant in her life, so his sudden absence would be difficult to accept. She could become withdrawn and start to question her self - worth.
Kakashi would be the most perceptive. He'd see how Naruto singing 'Monster' is not just a random act but a way for Naruto to connect with his own fears and insecurities. His friends from the Hidden Leaf Village would overall be proud of Naruto for trying something new and for using the song to share a part of himself that they may not have fully understood before.
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.
Sakura would probably be horrified. She knows Orochimaru's reputation for being cruel and power - hungry. She would immediately reject any offer from him and get ready to fight if necessary. After all, she has grown into a strong kunoichi and won't be easily intimidated.
Sakura might initially be terrified. She would probably try to fight back using her combat skills and medical ninjutsu to defend herself. Since she is a strong - willed character, she won't go down without a fight.
Sakura might be really touched. She would see Naruto in a new light, as someone who can express deep emotions not just through his actions but also through song. She might even join in, adding her own harmonies, showing her support for Naruto and also her own feelings towards those they've lost.