Output dropdown results into Woocommerce Order details

Take a look at the WooCommerce docs here: Tutorial – Customising checkout fields using actions and filters

your code would look something like this:

function my_custom_checkout_field( $checkout ) {

woocommerce_form_field( 'my_field_name', array(
    'type'          => 'select',
    'class'         => array('my-field-class form-row-wide'),
    'label'         => __('Question?'),
    'placeholder'       => __('Enter something'),
    'options'           => array(
        'Yes' => __('Yes', 'woocommerce' ),
        'No' => __('No', 'woocommerce' )
    )
    ), $checkout->get_value( 'my_field_name' ));
}

though the input also needs processing to be displayed in the WC admin. If you have little experience with PHP, it’s not that easy though. You could take a look at this plugin for inspiration.