This is a modifier error question. It is "Franklin Roosevelt" who could not walk without assistance, and not the "radio" as the orignal sentence indicates. The modifying phrase, "unable to walk without assistance" should be placed as close as possible to the subject -"Franklin Roosevelt" - it modifies. We can therefore narrow down our options to (B) and (C). (B) is incorrect because "conveyed in radio a message" is an unidiomatic construction. (C) is the best answer since it clearly explains that it was Franklin Roosevelt who used radio to convey a message.