電気がついているから、彼はもう帰っているに違いない。
Q. How certain is the speaker that he is home?
Translation
The lights are on, so he must already be home.
Explanation
〜に違いない expresses a confident inference from evidence: 'must be'. The lights-on observation grounds the near-certainty; it is not hearsay (D) or mere hope (C).