you don't seem to be using the entity type correctly. you need to specify an entity class. The class is of the entity you are displaying so you'll probably want the Responses class. You might also want to consider having the responses as a collection for each question so you can got $q->getResponses() instead of having to do a query for every field.
$formBuilder->add('rep'.$q->getId(), 'entity', array(
'class' => 'QuizzBundle:Responses',
'label'=>$q->getTitre(),
'property' => 'response_field_you_want_displayed',
'choices' => $q->getResponses(),
'multiple'=>false,
'expanded'=>true,
'required'=>true
));