If the selectbox element has an ID tag then you can use code below.

FeatureContext.php

use Behat\MinkExtension\Context\MinkContext;

class FeatureContext extends MinkContext
{
    /**
     * @Then /^The option "([^"]*)" should be selected in select box "([^"]*)"$/
     *
     * @param $elementId ID of the selectbox element.
     * @param $expectedOption What option should be verified. Data needs to exploded.
     * @throws \InvalidArgumentException
     */
    public function getSelectedSelectboxOption($expectedOption, $elementId)
    {
        $session = $this->getSession();
        $page = $session->getPage();

        $element = $page->find('css', '#' . $elementId);

        if (null === $element) {
            throw new \InvalidArgumentException(sprintf('Could not evaluate CSS element: "%s"', $elementId));
        }

        //index 0 = expected option value, index 1 = human friendly option text
        $options = explode('~', $expectedOption);

        $option = $element->getValue();

        if ($option != $options[0]) {
            throw new \InvalidArgumentException(sprintf('CSS element has different option selected'));
        }
    }
}

#behat #css