You have to add attribute to the customer not Column to the Database.
Please refer to this article to add attribute to the customer in correct way,
https://webkul.com/blog/how-to-create-customer-custom-attribute-in-magento-2-0/
in this article they added text attribute, you need to add select type instead for your case
when you create the attribute in the correct way, you will find it in the admin customer page and will be saved automatically as any other value.
for sending email you need to use the events, there is event (customer_save_after)
You have to check the original value and the new value, if the values are not the same that mean you changed the value of this attribute, if the value is changed then you can send email to the customer
it should be like this
{
$oCustomer = $observer->getCustomer();
$origin_data = $oCustomer->getOrigData("{your_attribute}");
$new_data = $oCustomer->getData("your_attribute");
// send email here
}
Hope this will help you