What is a necessary action to give edit access to a user in the User lookup field on a Private Employee Review object?

To grant edit access to a user in the User lookup field on a Private Employee Review object, creating an Apex trigger to insert an Employee Review Share record with an access level of Edit is necessary.

In a Private sharing model, records are not shared with users by default. When using custom objects, sharing settings control access. Simply populating the User lookup field does not automatically grant access—it requires deliberate action to facilitate that access when the sharing model restricts it.

An Apex trigger allows for more granular control over the sharing of records. By writing a trigger that responds to the creation or modification of Employee Review records, you can programmatically create a sharing record (Employee Review Share) that designates the specified user and sets their access level to Edit. This ensures that the user can not only view but also make changes to the Employee Review record, fulfilling the requirement for edit access.

The other choices, while they involve sharing in some form, do not align with the capabilities or requirements of the Salesforce security and sharing model in this scenario. Therefore, using Apex provides the necessary flexibility and functionality to meet the requirement effectively.