Okay, your issue is 1 of 2 likely places. 1) $row['txtid'] is blank, thus the value of your drop down option is blank. 2) the data isn't getting to sent.php or being read properly in sent.php.
So start with your dropdown. View the Source of the page and make sure the drop down actually has a value (that isn't spaces/blank).
Then submit the form after adding var_dump($_POST); to your sent.php so you can see what data is being received by sent.php.
From what you've done so far and what you have provided us, my guess is $row['txtid'] doesn't contain what you think it does.