List<AggregateResult> ars = [SELECT YTPQuiz__c quiz FROM YTPQuestion__c GROUP BY YTPQuiz__c HAVING Count(Id) > 1];
System.debug(ars);
for (AggregateResult ar : ars) {
    System.debug(ar.quiz);
}

DEBUG|AggregateResult:{quiz=a1XY0000000012}
DEBUG|AggregateResult:{quiz=a1XY0000000013}
DEBUG|AggregateResult:{quiz=a1XY0000000014}
DEBUG|AggregateResult:{quiz=a1XY0000000015}

DEBUG|AggregateResult:{YTPQuiz__c=a1XY0000000012}
DEBUG|AggregateResult:{YTPQuiz__c=a1XY0000000013}
DEBUG|AggregateResult:{YTPQuiz__c=a1XY0000000014}
DEBUG|AggregateResult:{YTPQuiz__c=a1XY0000000015}