Final answer:
The Security and Access feature that allows Service Reps to grant access to Cases to other Salesforce users is Manual Sharing.
Step-by-step explanation:
The Security and Access feature that can fulfill the Call Center Manager's requirement of allowing Service Reps to manually grant access to Cases to other Salesforce users is Manual Sharing.
Manual Sharing is a feature in Salesforce that allows users to share records with other users manually. With Manual Sharing, Service Reps can grant access to Cases by manually sharing them with other Salesforce users.
To enable Manual Sharing for Cases, the Call Center Manager can customize the Case page layout to include the Manual Sharing button. This button will allow Service Reps to share the Case with specific users or public groups in Salesforce.
Learn more about Manual Sharing