An electronic communication that is usually written by a single author and devoted to a single subject is option C) “Internet weblogs”. Weblogs are posts or entries published on the internet. They are told by a single person and cover one topic.
Option A is incorrect since radio broadcasts are radio waves transmissions targeted to wide audiences, they can be transmitted by several people simultaneously in a radio program and during this program several topics can be discussed, usually with the active participation of the audience.
Option B is also incorrect as a TV show usually has many participants and several topics are discussed.
And finally option D is also incorrect since magazines usually cover several topics in their articles and these articles are written by different authors.