Final answer:
A website might work online but not on localhost typically due to missing server-side functionalities, reliance on external APIs, or absolute paths in the code. Firewall restrictions could also hinder local operation if they block necessary components.
Step-by-step explanation:
When your website works online but not offline on your localhost, the issue is likely not related to limited server capacity (A) or internet connection issues (B) since those would affect the website's online performance, not offline. The problem is probably not due to browser compatibility (D) either, as the same browser would be used in both cases. The most likely cause from the options provided would be (C) firewall restrictions which could potentially block certain aspects of the site from functioning properly on a local server.
However, more common reasons for a website to work online but fail to run properly offline include missing server-side functionalities that are not replicated on the local environment, reliance on external APIs that aren't accessible without an internet connection, or absolute paths in the code that only reference the live version of the website. To address these issues, you would need to replicate the server-side environment locally, adapt your code to work with relative paths, or ensure access to any external APIs if needed.