Often a user will leave your infinite-scroll feed to take an action: maybe to favorite an item or post a comment. From there, they'll hit back (on desktop), swipe back (iOS) or use the hardware back button (Android).

Q: Where do they end up?

The answer is obviously b), unless you really hate your users. Sadly, ) is often the case ecommerce sites, when browsing a long list of products.

Although the technical implementation details can be challenging, it's worth putting in the effort to avoid disorentating users. Viewing a product then navigating back should always return the user to the point where they left off.