Try it
add_filter('woocommerce_billing_fields', 'custom_woocommerce_billing_fields');
function custom_woocommerce_billing_fields($fields)
{
$fields['billing_options'] = array(
'label' => __('Ordering Method', 'woocommerce'), // Add custom field label
'placeholder' => _x('Your NIF here....', 'placeholder', 'woocommerce'), // Add custom field placeholder
'required' => true, // if field is required or not
'clear' => false, // add clear or not
'type' => 'radio', // add field type
'class' => array('form-group'), // add class name
'options' => array(
'privato_cittadino' => 'Privato cittadino',
'azienda_professionista' => 'Azienda o libero professionista',
),
);