Stub conduits out of the panel into the tray would be the best. Personally I like conduit for power especially. Tray is good for data cables, when there are a bunch of them. I'd go to a electrical contractor and see if they'd sell you some. The company I work for has tons of it just laying...
Really, a 100 amp panel with more spaces would have probably been plenty. Especially since it's a rental.
Unless there has been a problem with the main tripping.
I would go with figure A. The wire will have a little bit of movement with the ground, and the conduit won't, since it's strapped in place. Not sure if I can really explain it right, but an intructor in my apprenticeship explained why figure B is a bad idea.