In which t book does momiji protect tohru from akito?

Momiji doesn't protect Tohru from Akito in any of the Fruits Basket manga or anime. He's actually quite scared of Akito himself and often tries to avoid him.

The closest thing to Momiji protecting Tohru from Akito is in the Fruits Basket: Another manga. In that story, Momiji does intervene to protect Tohru from Akito's outburst, but it's not a direct confrontation. He calms Tohru down and helps her escape the situation.

It's important to remember that Momiji is a very gentle and empathetic character. He wouldn't physically confront Akito, even to protect Tohru. His actions are more about protecting her feelings and ensuring her safety.