C.

The relationship between Wonder Woman and Steve Trevor has had several iterations in the comics. Since the death of Steve Trevor is never a major point in Wonder Woman's story, I imagine they'll find a way around it. In the Justice League three-episode arc "The Savage Time," the League goes back in time to stop Vandal Savage's machinations and Steve Trevor winds up rescued from a mission that would have otherwise killed him. Diana and a much older Steve reconnect after the League returns to the present time.

Since Justice League (2017) will feature the Flash, it's possible they'll do something similar and use speed-force-time-travel shenanigans to bring Steve back into the fold.

C.

The relationship between Wonder Woman and Steve Trevor has had several iterations in the comics, so let's go through your three options.

A: he's definitely dead. There is no canon reason to think this. The death of Steve Trevor has never been an "accepted" part of Wonder Woman's story (unlike, say, Spider-Man with the death of Uncle Ben, or Batman with the deaths of his parents).

B: he's definitely alive. There is no canon reason to think he's currently alive. The airplane explosion has never been shown before (to the best of my knowledge), so there is no precedent for him coming back from it.

This leaves C: he's presumed dead, but will probably come back. There is some precedent for this: In the Justice League three-episode arc "The Savage Time," the League goes back in time to stop Vandal Savage's machinations and Steve Trevor winds up rescued from a mission that would have otherwise killed him. Diana and a much older Steve reconnect after the League returns to the present time.

Since Justice League (2017) will feature the Flash, it's possible they'll do something similar and use speed-force-time-travel shenanigans to bring Steve back into the fold.