Marine ply the only way to go if you want it to last
forever. But then you'd need to epoxy coat both sides, glass the top with a light glass, seal the edges very very well, and keep it painted to protect the epoxy. Any and all penetrations into the plywood also need epoxy sealing. Ideally screwholes for example should be overdrilled and filled with epoxy putty and then the screws drilled into putty.
Will add a significant amount of cost and labour. But will outlast you if done right. Marine plywood is untreated and will not last in an exterior application if unprotected. The value is in the good bending properties, high quality wood, and lack of voids in the laminations.
If you only want it to last 10-20 years then treated ply would work. You should still seal the edges very well all around and coat both sides to stop checking. Epoxy might not bond to it, depending on moisture content and what treatment has been used. I have however managed to epoxy coat H5 treated wood (boat trailer skids) so it can be done. IF the epoxy bonds well then this will also last as long as you. I have used a roll on bedliner (
Nutech BullyLiner - Fisher Group (duraspec.co.nz)) to coat my sole. Seems very tough and was easy to apply. Also waterproof, you might get away with just this on treated ply.
I'd go for option B myself on that boat. I'd still strongly recommend that you get a small pack of epoxy glue and use it to seal all edge grain and screwholes everywhere. Then a couple of light coats on the surfaces. Epoxy makes an excellent primer and works better than sealant on wood and cheaper too.