I would go with C as well, if you COULD use someone else's arena. Young ones like Aires need room to goof off, and plenty of it :) the only thing I'd worry about is REALLY staying on top of his worming schedual if he's in the same pasture as a bunch of others (boarders, perhaps) because otherwise he might pick something up. Vaccinations too.
I think I'm packing my two girls up and moving to AZ though, the cheapest board that we have in my area is $250 for pasture board with other horses, and they're minis so I can't do that! No seperate feeding either- just 12% sweet feed pellets and a shared round bale for 5-7 horses every three days.