The field attribute that should be selected are Unique and External ID. Unique means that the field will not allow duplicate values. External ID means that the field can be used as a foreign key for data imports and upserts. Option B is incorrect because Required means that the field must have a value and cannot be left blank, which is not necessary for this requirement. Option D is incorrect because Text (encrypted) means that the field will store sensitive data in an encrypted format, which is not necessary for this requirement.