He could initially be confused and think it was some sort of strange jutsu gone wrong or a prank by one of his friends other than Temari. But when he realizes it was Temari, he might become a bit flustered and try to act cool while secretly being very embarrassed. He might also try to find a discreet way to return it to her as soon as possible to avoid any more awkwardness.
Naruto would probably be really shocked at first. He might blush furiously and be at a loss for words. After all, it's not an everyday occurrence to find a bra in your pocket.
Perhaps it's part of a prank plot within the fanfic. Temari, being a bit mischievous in this story, decides to play a joke on Naruto by putting her bra in his pocket. This then sets off a chain of events where Naruto becomes the center of attention among his friends who notice something strange in his pocket, leading to a lot of laughter and banter.
I'm not aware of the popularity of this specific fanfic. There are so many Naruto fanfics out there that it's hard to say whether this one has gained a large following.
It might be a fan - created scenario where the author wanted to add an unexpected and comical element to the Naruto universe. Maybe it's part of a larger story about the relationship between Temari and Naruto, with this act being a catalyst for some sort of misunderstanding or adventure. However, without reading it, it's hard to say for sure.
Well, Naruto's friends' reactions would depend on their personalities. Sasuke, if he's around, might seem indifferent on the surface but secretly think that Naruto is being an idiot as usual. Hinata might be a bit shy about the whole situation, but also happy for Naruto if she senses that he really likes Temari. And Lee would probably be super excited and start shouting words of encouragement to Naruto, saying things like 'Go for it, Naruto! Show your youthfulness in this pursuit of love!'
Sakura might find out about it later. She would probably tease Naruto about it, making him even more flustered. As for Sasuke, he might just roll his eyes at the whole situation, thinking it's just another one of Naruto's silly misadventures. But if the story is set in a more serious context, Sasuke could also see it as a sign of Naruto's growing closeness to Hinata and might have some complex thoughts about it.
Sakura would probably be very angry at first. She'd likely try to find out the truth immediately and would not rest until she clears Naruto's name. She might use her medical ninjutsu skills to get into the jail if needed.
In Konoha, Naruto's village, there would likely be a great deal of celebration. Naruto is a hero in Konoha, and the news of him having a child would be seen as a sign of new hope and the continuation of a strong bloodline. The villagers might throw parties and offer gifts for the soon - to - be - parents. In Suna, Temari's village, there would also be celebration but with a bit more practicality. The Sabaku clan would be proud, and they might start making plans to strengthen the alliance with Konoha through this new connection. There could also be some concern in Suna about Temari being in Konoha during her pregnancy, but overall, it would be a positive reaction.
Naruto would probably be extremely hurt. He has always considered Hinata a friend and perhaps more. Betrayal from her would be a huge blow to his trusting nature. He might initially be in denial, not wanting to believe that Hinata could do such a thing.
Naruto would be deeply hurt. He would probably feel a mix of anger and sadness. At first, he might try to reason with Tsunade, to find out why she betrayed him.
At first, Naruto would be shocked and confused. His world view would be shattered. But then, he would draw on his inner strength. He has overcome so many hardships before. He might feel a deep sadness, but also a sense of resolve. He would still believe in the power of love and friendship, and use that to face Kushina's betrayal. He could also use this experience to grow even stronger and more determined to prove himself and his beliefs.