nobody can stop me from carrying out this plan,---?

nobody,“没有一个人”,表示单数概念, 所以反义疑问句可以用A,因为一般这种所指都用男性he
类似的“人类”,这个词常用man(虽然人类有男有女)
但是,现代英语强调语言使用要性别平等,比如nobody can force his opinions on others,这个以前是常见的,但现在常见的就是nobody can force their opinions on others或者nobody can force his or her opinions on others
类似的everyone is entitled to their own freedom of choice.
所以这一题A B都可以,选A是传统用法,选B就是现代用法,实际应用中两者都可以,都有人用