Option A isn't going to be very strong, it has almost no vertical bracing. Essentially all the stiffness would come from the bolts through the side plates into the profile.

Option B is ok but Option C is going to be a bit stronger. If you want the space internally to mount the stepper then you could go with option A but put a 10 or 15mm plate across the back (I assume the sloped side is the front). You should really have a look through some of Jazz's threads if you want to see a good design that keeps everything internal, the design I'm thinking of has a 20mm plate a the front of the gantry with a slot machined out of it for the screw connection. It served as inspiration for my design.

If you want to do a quick experiment get a couple of lengths of 2x4 timber and nail them to some ply in roughly the design of option A, it'll take 5 minutes to build and you'll immediately see why it won't work as a gantry as it'll wobble around like a jelly. Grab yourself a bit of hardboard and nail it across the back, you could probably stand on it them.