The ribcage is just some simple geometry paired with an off-white emissive material.

The shadow body has the slightest bit of transparency allowing the heart & and ribcage to passthrough. Shouldn't really cause issues at such a low poly count.

This project plays with light and dark as a core mechanic. So the exposed ribcage & heart is to have a possible giveaway when they are in the dark. Might be less visible in-engine, don't know just yet.

Really depends on what vibe you aiming for I suppose. Personally, I think option B best fits your tone best. Especially if you game is in the inspired by titles like Farcry II and Stalker.