What kind of fossil would a gastropoda shell must likely be?

You didn't give any answer choices (i'm answering this through research and what i already know)

the first is fossilized dung, and that's not generally gastropod shells. A carbon film would be something thin, seeing as it's a film, and gastropods aren't that. A gastrolith is a stone swallowed by an animal to help with digestion, and that's also not a gastropod. 

Gastropod fossils may be molds, but usually aren't. Still, they can be. It's a better answer than; film or stomach stones.