The correct SOQL query to return a unique list of Contact records that have no 'Fulfilled' Orders is one that selects Ids from the Contact object where the Contact Id is not in the set of Contact Ids related to 'Fulfilled' Orders. This is done by a subquery that selects the Contact__c field from the Order__c records where the Status__c is 'Fulfilled'.References: SOQL and SOSL Reference Guide - Subqueries