roles.php

add_action('init', function(){
     global $wp_roles; 
     $roles = $wp_roles->roles; 

     $choices = array();

     foreach ( $roles as $key => $value ){
        if ( $key !== 'administrator')
           continue;

       $caps = $value['capabilities'];

       foreach ( $caps as $cap_key => $cap_value )
            $choices[] = $cap_key;
     }

     var_dump($choices);
 });