I was doing the Dawnguard questline and I was doing this quest called PHOPHET, and when I get to a point in the quest where it says "Talk to the moth priest.", And hes in dawnguard, And I talk to him and he only has 2 options, making it impossible to progress in the storyline.
Im playing on Skyrim Legendary Edition, on PC no mods.
On the Unofficial Elder Scrolls Pages wiki page for this quest, one of the bugs listed is what seems to fit your description. A console command that might work would be to type in this console command, which would set the quest stage back one. More info on which specific stage to set it to can be found on the page that was linked. (Note: It is highly recommended to save before using any console commands):
setstage DLC1VQ03Hunter 80
If you don't want to use console commands, the only other option would be to load an earlier save game, before you ran into this problem. However, there is no guarantee that this will work.