Magento 2: How add dropdown in admin form?

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;
    }
}

I assume you found this out by now, but the xml in the form ui

<item name="options" xsi:type="string">Book\Flip\Model\Category\Attribute\Source\Category</item>

belongs under data as a sibling of config not a child, and its xsi:type should be "object"

Like this

<argument name="data" xsi:type="array">
    <item name="options" xsi:type="object">Book\Flip\Model\Category\Attribute\Source\Category</item>
    <item name="config" xsi:type="array">
        <item name="dataType" xsi:type="string">text</item>
        <item name="label" translate="true" xsi:type="string">category</item>
        ...

I also was stuck on this issue for awhile until I figured out that bit.

class category implements \Magento\Framework\Option\ArrayInterface
{ 
    //Below function is supposed to return options.
    public function toOptionArray()
    {
        return [
            ['value' => 1, 'label' => 'label1'],
            ['value' => 2, 'label' => 'label2']
        ];
    }
}