Unchecking the Grant Access Using Hierarchies checkbox in Sharing Settings for the Complaints object and creating a criteria-based sharing rule that grants Read/Write access to the Complaints Specialist public group are two options that can meet the requirements of restricting access to Complaint records to only users within the Complaints Specialist public group. Grant Access Using Hierarchies is a setting that determines whether users above a record owner in the role hierarchy can access that record. By unchecking this setting for the Complaints object, the consultant can prevent managers or other users higher in the role hierarchy from viewing or editing Complaint records that they do not own. A criteria-based sharing rule is a rule that grants access to records that meet certain criteria, such as field values, record types, etc. By creating a criteria-based sharing rule that grants Read/Write access to the Complaints Specialist public group, the consultant can ensure that only users in this group can view and edit any Complaint record, regardless of who owns it.