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.
A. Marconi's conception of the radio was as a substitute for the telephone, a tool for private conversation; instead, it is
B. Marconi conceived of the radio as a substitute for the telephone, a tool for private conversation, but which is
C. Marconi conceived of the radio as a tool for private conversation that could substitute for the telephone; instead, it has become
D. Marconi conceived of the radio to be a tool for private conversation, a substitute for the telephone, which has become
E. Marconi conceived of the radio to be a substitute for the telephone, a tool for private conversation, other than what it is,

In choice B, which is the reference for the relative pronoun "which"? I know that "which" refers to the noun next to it (in this case, "conversation"). However, reading other forums, some experts mention that it refers to "a tool for private conversation" because there is a "but", so there should be parallelism.

Also, other expert mentions that it refers to "telephone". So, there are many and diverse explanations! What is correct?, please your help.