Criteria-based sharing rules are the best tool to use for automatically sharing records based on specific criteria, such as the opportunity type being "Service Agreement." This rule can share opportunities with all users in a particular role, such as the Service Manager role, when the criteria are met, thereby adhering to the private sharing model while still providing necessary access.
A: Owner-based sharing rules focus on the record owner's role or group, not the record type.
C: Apex sharing is used for more complex scenarios not easily handled by declarative sharing settings.
D: Manual sharing is not scalable or automatic.
Reference: Using Criteria-Based Sharing Rules on Salesforce Help