ImageStyleDeleteForm::form

public ImageStyleDeleteForm::form(array $form, FormStateInterface $form_state)

public function form(array $form, FormStateInterface $form_state) {
  $replacement_styles = $this->getReplacementOptions();
  // If there are non-empty options in the list, allow the user to optionally
  // pick up a replacement.
  if (count($replacement_styles) > 1) {
    $form['replacement'] = [
      '#type' => 'select',
      '#title' => $this->t('Replacement style'),
      '#options' => $replacement_styles,
      '#empty_option' => $this->t('- No replacement -'),
      '#weight' => -5,
    ];
  }

  return parent::form($form, $form_state);
}