Marconi’s conception of the radio was as a substitute for the telephone, a tool for private conversation; instead, it is precisely the opposite, a tool for communicating with a large, public audience.

1. Marconi’s conception of the radio was as a substitute for the telephone, a tool for private conversation; instead, it is
2. Marconi conceived of the radio as a substitute for the telephone, a tool for private conversation, but which is
3. Marconi conceived of the radio as a tool for private conversation that could substitute for the telephone; instead, it has become
4. Marconi conceived of the radio to be a tool for private conversation, a substitute for the telephone, which has become
5. Marconi conceived of the radio to be a substitute for the telephone, a tool for private conversation, other than what it is,

i would translate the sentence into chinese, then you will see.

马科尼曾经考虑把无线电作为一种可以替代电话的私人交谈工具；但事实上恰恰相反，无线电成为了大众传媒工具。

C表达的意思是最通顺的。

E的表达有两处非常蹩脚的地方，尤其是“other than what it is”