C.to help 为正确选项.
with Mr Brown to help us是with 的复合结构,属于“with+名词或代词+非谓语”的一种形式,做状语.根据句意,help的动作还没发生,故用不定式.
【俊狼猎英】团队为什么不用helping请比较用helping和to help的区别：With Mr Brown helping us,we have made great progress in our oral practice. With so much homework to do,Tom couldn't go to the cinema.With the homework done,Tom went to the cineam。【俊狼猎英】团队不可以理解成，Mr Brown 正在帮助我，我的口语将会有进步吗？不可以，那是中国的思维方式。 【俊狼猎英】团队