How to make MultiCheckBox Label bold or strong?

I have the following multicheckbox:

   $this->add(array(
        'type' => 'DoctrineModule\Form\Element\ObjectMultiCheckbox',

        'name' => 'directPractice',
        'options' => array(
           'label' => 'A. Check all direct practice field education assignments',

         **EDIT 1:**
         'label_attributes' => array(
                'class'  => 'label-multicheckbox-group'
            ),


            'object_manager' => $this->getObjectManager(),
            'target_class' => 'OnlineFieldEvaluation\Entity\FieldEducationAssignments', //'YOUR ENTITY NAMESPACE'
            'property' => 'directPractice', //'your db column name'
            'value_options' => array(
                '1' => 'Adults',
                '2' => 'Individuals',
                '3' => 'Information and Referral',
                '4' => 'Families',
            ),
        ),
        'attributes' => array(
            'value' => '1', //set checked to '1'
            'multiple' => true,
        )
    ));

How to make the following part bold? Using label_attributes makes all labels bold, and I want just the main label for the multibox be bold.

'label' => 'A. Check all direct practice field education assignments',

EDIT 1: add label_attributes to "options"

When I add label_attributes as @itrascastro suggested, all labels become bold and I want only the top one to become bold: multicheckbox

Since there is no build in option to do this just for the top label, I used jQuery to accomplish this:

$("label[for]").each(function(){
    $(this).addClass("label-bold");
});