His favorite authors include J.R.R. Tolkien, C.S. Lewis and Ursula K. Le Guin all masters of speculative fiction.
Correct: B
First, for the list of authors, an Oxford comma is needed before 'and Ursula K. Le Guin'. Second, the phrase 'all masters of speculative fiction' is an appositive that renames the list of authors. This appositive should be set off by a comma, or an em dash for emphasis, as it provides additional, non-essential information. Option B correctly uses the Oxford comma in the list and then sets off the non-essential appositive with another comma.