This answer meets the requirement of improving the performance of the cache hit ratio associated with the image file. The reason is:
Custom cache keys allow you to control which parts of the request URL are used to build the cache key. The cache key is a unique identifier that Cloud CDN uses to store and retrieve cached content1.
By default, Cloud CDN uses the complete request URL, including the protocol (http or https) and the host (the domain name), to build the cache key. This means that if the same image file is requested from different domains or protocols, Cloud CDN will cache multiple copies of it, which reduces the cache hit ratio1.
By clearing the Host and Protocol checkboxes, you can tell Cloud CDN to ignore these parts of the request URL when building the cache key. This way, Cloud CDN will cache only one copy of the image file, regardless of which domain or protocol it is requested from, which improves the cache hit ratio1.
Option B is incorrect because configuring Cloud Storage as a custom origin backend does not affect the cache hit ratio. It only affects how Cloud CDN retrieves the content from the origin if it is not cached. Option C is incorrect because configuring versioned URLs for each domain does not improve the cache hit ratio. It actually worsens it, because it creates more variations of the request URL that Cloud CDN has to cache separately. Option D is incorrect because configuring the default TTL as 0 for the image file means that Cloud CDN will not cache it at all, which defeats the purpose of using Cloud CDN.
Reference:
Custom cache keys | Cloud CDN | Google Cloud