The best approach to ensure the assigned reviewer obtains Read/Write access to the record is:
C. Create an After Insert trigger on the Convention Attendee custom object, and use Apex Sharing Reasons and Apex Managed Sharing.

Apex Managed Sharing allows developers to programmatically share custom objects. When you use Apex managed sharing to share the custom object, only users with the “Modify All Data” permission can add or change the sharing on the custom object’s record, and the sharing access is maintained across record owner changes.

Apex Sharing Reasons are used to define custom sharing reasons for Apex managed sharing. They allow you to group different types of shares for the same record and distinguish between them. For example, you can create an Apex Sharing Reason called "Event Reviewer" and use it to share the records with the users who are assigned as event reviewers.

To use Apex Managed Sharing and Apex Sharing Reasons, you need to create a trigger on the custom object that inserts or updates share records for the records that need to be shared. The trigger should be an After Insert trigger, because you need to have the record ID of the inserted record before you can share it.

The other options are not correct for the following reasons:

- Option A is incorrect because criteria-based sharing rules cannot grant Read/Write access to individual users based on a lookup field. They can only grant access to predefined groups of users, such as roles, public groups, or territories.
- Option B is incorrect because a Before Insert trigger cannot access the record ID of the inserted record, which is required for creating share records.
- Option D is incorrect because it is a repetition of option A.

https://www.forcetalks.com/salesforce-topic/what-is-apex-managed-sharing-2/
https://developer.salesforce.com/docs/atlas.en-us.apexcode.meta/apexcode/apex_bulk_sharing.htm...