First

<?php $mb->the_field('first_option_discount'); ?>
<select name="<?php $mb->the_name(); ?>">
    <option value=""></option>
    <option value="+"<?php $mb->the_select_state('+'); ?>>+</option>
    <option value="-"<?php $mb->the_select_state('-'); ?>>-</option>
</select>