Example of how to add a custom column to the EE registration admin list tables, this examples pulls the answer to question ID 11 and outputs that in a 'Custom Reg Question' column.

example.php

<?php //Please do not include the opening PHP tag if you already have one

function tw_custom_columns( $columns, $screen ) {
    // This is for the 'default' registration list table
    // Event Espresso -> Registrations.
    if($screen == 'espresso_registrations_default'){
        // EEH_Array::insert_into_array() allows you to specific a specific key
        // that you want to add your additional column before/after.
        // This adds the custom-reg-qst-column just before 'actions' column.      
        $columns = EEH_Array::insert_into_array(
            $columns,
            array( 'custom-reg-qst-column' => 'Custom Reg Question' ), 
            'actions'
        );
    }
    //Add a custom-reg-qst-column column to the contact list table.
    if($screen == 'espresso_registrations_event_registrations') {
        //This is another method you can use that just adds the column to the end of the array.
        $columns['custom-reg-qst-column'] = 'Custom Reg Question';
    }
    return $columns;
}
add_filter('FHEE_manage_event-espresso_page_espresso_registrations_columns', 'tw_custom_columns', 10, 2);

function tw_checkin_table_custom_question( $item, $screen ){
    //Sanity check to confirm we have an EE_Registration object.
    if($item instanceof EE_Registration){ 
      //Pull the answer object for Question ID 11 using the current registration.
      $question_id = 11;
      $answer_obj = EEM_Answer::instance()->get_registration_question_answer_object( $item, $question_id );
      if ( $answer_obj instanceof EE_Answer ){
        //If we have an answer object, echo the 'pretty' value for it.
        echo $answer_obj->pretty_value();
      } 
    }
}
// 'custom-reg-qst-column' in the action name here needs to be changed to match your column name set in the function above.
add_action( 'AHEE__EE_Admin_List_Table__column_custom-reg-qst-column__event-espresso_page_espresso_registrations', 'tw_checkin_table_custom_question', 10, 2 );

//Enable sorting for the custom column.
function tw_ee_sortable_columns( $sortable_columns, $screen ) {

    if ( $screen == 'espresso_registrations_default' ) {
        $sortable_columns['custom-reg-qst-column'] = array('custom-reg-qst-column' => false);
    }
    return $sortable_columns;
}
add_filter('FHEE_manage_event-espresso_page_espresso_registrations_sortable_columns', 'tw_ee_sortable_columns', 10, 2);

// The models only accept certain values for orderby, normally the column name is used but if thats custom we'll need to pass a value
// the models understand. THis sets the order by to 'Answer.ANS_value' when you select the custom column above.
function tw_ee_get_orderby_for_registrations_query( $order_by, $request)
{   
    if( isset($order_by['custom-reg-qst-column'] ) ) {
        $fixed_order_by = array(
            'Answer.ANS_value' => $order_by['custom-reg-qst-column']
        );
        unset($order_by['custom-reg-qst-column']);

        foreach( $order_by as $key => $value) {
            $fixed_order_by[$key] = $value;
        }

        return $fixed_order_by;
    }

    //Return the original order_by array.
    return $order_by;
}
add_filter('FHEE__Registrations_Admin_Page___get_orderby_for_registrations_query', 'tw_ee_get_orderby_for_registrations_query', 10, 2);