D. Create 'Promotion__Share' records and use a sharing group associated with the 'Sales Agent' role to grant access.

Apex Managed Sharing can be used to create a share record for a custom object. The sharing group associated with the 'Sales Agent' role can be used to specify the UserOrGroupId for creating the share records for this requirement.
It's easier to use a sharing group since that already exists in the group maintenance tables. Workbench can be used to create a SOQL query associated with a particular sharing group by querying the 'Group' object.
Also, it's important to note that there is a difference between sharing groups and share groups. Unlike sharing groups, share groups are used to share records owned by the high-volume community and portal users with internal and external users. On the other hand, sharing groups are automatically created for roles by Salesforce.

https://developer.salesforce.com/docs/atlas.en-us.apexcode.meta/apexcode/apex_bulk_sharing_creating_with_apex.htm...