我觉得这道题可以还原一下：(变成强调句)
It was for your encouragement that I invented this machine,强调句 强调部分为for your encouragement.对此句话进行虚拟,即对过去时进行虚拟,“如果不是因为你的鼓励我不会发明这个机器.”即“If it had not been for your encouragement,I'd never invented this machine”.省略if ,将had提前,整句变为：Had it not been for your encouragement,I'd never have invented this machine.所以答案为D.C选项的句子结构还原就是不正确的
不知这样解释你懂不懂.