Magento – Magento 2: How to add websites dropdown in admin form

For field dependency in admin form, try below code

$selectField = $fieldset->addField('select_field', 'select', array(
    'label' => 'Show/Hide Selection',
    'name' => 'select_field',           
    'values' => array(
        array(
            'value' => 1,
            'label' => 'Yes',
        ),
        array(
            'value' => 0,
            'label' => 'No',
        ),
    )
));
$hideField = $fieldset->addField('field_to_hide', 'text', ['name' => 'field_name','value'=> 'your_value', 'label' => __('Your label Name')]
));
//code to dependence field
$dependence = $this->getLayout()->createBlock(
    'Magento\Backend\Block\Widget\Form\Element\Dependence')
    ->addFieldMap($selectField->getHtmlId(), $selectField->getName())
    ->addFieldMap($hideField->getHtmlId(), $hideField->getName())
    ->addFieldDependence(
        $hideField->getName(),
        $selectField->getName(),
        '1'
    );
$this->setChild('form_after', $dependence);

Change Category class by following code:

namespace Book\Flip\Model\Source;

class Category implements \Magento\Framework\Option\ArrayInterface
{
    /**
     * Retrieve options array.
     *
     * @return array
     */
    public function toOptionArray()
    {
        $result = [];

        foreach (self::getOptionArray() as $index => $value) {
            $result[] = ['value' => $index, 'label' => $value];
        }

        return $result;
    }

    /**
     * Retrieve option array
     *
     * @return string[]
     */
    public static function getOptionArray()
    {
        return [1 => __('science')];
    }

    /**
     * Retrieve option array with empty value
     *
     * @return string[]
     */
    public function getAllOptions()
    {
        $result = [];

        foreach (self::getOptionArray() as $index => $value) {
            $result[] = ['value' => $index, 'label' => $value];
        }

        return $result;
    }

    /**
     * Retrieve option text by option value
     *
     * @param string $optionId
     * @return string
     */
    public function getOptionText($optionId)
    {
        $options = self::getOptionArray();

        return isset($options[$optionId]) ? $options[$optionId] : null;
    }
}