How to Display Custom Field Values in WordPress Admin for Custom Post Types

functions.php

<?php

// ... Rest of functions.php

/**
 * Add the question code column to the question custom post type.
 */
function add_question_code_column_to_question_cpt($columns) {
    $columns['question_code'] = 'Question Code';
    return $columns;
}
add_filter('manage_question_posts_columns', 'add_question_code_column_to_question_cpt');

function question_code_column_content($column, $post_id) {
    if ($column == 'question_code') {
        $question_code = get_post_meta($post_id, 'question_code', true);
        echo $question_code;
    }
}
add_action('manage_question_posts_custom_column', 'question_code_column_content', 10, 2);