The correct answer is A. Configure a Cloud Storage bucket permission that gives the Storage Legacy Object Reader role.

This answer is based on the following facts:

Cloud CDN can serve private content from Cloud Storage buckets, but you need to grant the appropriate permissions to the Google-managed service account that represents your load balancer1.

The Storage Legacy Object Reader role grants read access to objects in a bucket2.

The Cache-control: private header indicates that the object is not publicly readable and requires authentication3.

The USE_ORIGIN_HEADERS cache mode instructs Cloud CDN to cache responses based on the Cache-Control and Expires headers from the origin server4. Changing the cache mode, increasing the TTL, or enabling negative caching will not affect the 403 error.