Can a ranger solo any of the dungeons?
Sure, any of them that don’t require group mechanics to pass. You need to know the path really well and be a well geared level 80.
I’m not sure where you are in the game; if this is your first character, the answer is pretty much a hard “no”, and they’re meant to be done by five players.
I don’t imagine it would be worth the time or effort it would take. I’ve soloed keeps in WvW on several occasions and it is always extremely boring because “boss fights” in this game are just you beating on a big blob of health for half an hour and avoiding the ridiculously telegraphed hits.
Besides a very few dungeon paths with mechanics that requires more than 1 player, all the others are soloable by every class, what changes is how long it will take and how easy.
A time ago I wanted to test a new build for my ranger and for that I went to a lvl 1 Snowblind fractal solo and see how long I would last, in the ended I was able to finish it.
(and the other 8 elite specs maxed too)