Well, in some fanfictions, Fu might use her unique jutsu skills. For example, she could create a powerful barrier to protect Naruto from Sakura if Sakura is somehow being controlled or acting aggressively. Fu's ability to manipulate nature energy could also play a role. She might use it to disorient Sakura or make her lose her focus on Naruto.
In many fanfictions, Naruto might use his ninja skills. He could suddenly appear and physically stop the bullies. For example, he might use his Shadow Clone Jutsu to surround the bullies and intimidate them into leaving Sakura alone.
Fu could use her healing abilities. If Sakura is attacking Naruto with some kind of harmful jutsu that causes internal damage, Fu can use her healing powers to counteract the effects. She could also transfer some of her own life force or energy to Naruto to keep him alive and stable while dealing with Sakura. This would be a more spiritual or mystical way of saving Naruto.
Well, Naruto has a strong sense of justice. He can't stand to see anyone being bullied, especially someone he cares about like Sakura. He might also want to prove himself. By standing up to the bullies, he shows that he's not the weakling some people think he is.
It could be Naruto. Naruto has a strong bond with Sakura and would sense that something was wrong. He'd probably track her down using his amazing sense of smell or by asking around. Once he found Sasori, Naruto would use his Rasengan and his newfound Sage Mode powers to take on Sasori and rescue Sakura.
Naruto could be the one to save her. He has a strong bond with Sakura and would never let her be in danger for long. His powerful ninja skills and his unwavering determination would make him a likely candidate to rescue her from Black Zetsu's grip.
In many Sakura Naruto fanfics, Sakura is often depicted as a strong - willed character. She uses her medical skills not only for healing but also for strategic purposes during battles.
One possible reason could be Naruto's natural sense of justice. He can't stand to see a child in danger, and Koyuki being a child at that time, Naruto's heroic nature kicks in. He has always been the type to help those in need.