For those who don’t know about data validation, it’s purpose is to limit the type of data users could enter into a cell. If a data validation is a list, it will only allow users to select from the list.
Copy the formula “=OFFSET(A1,0,0,COUNTA($A$1:$A$100),1)” into the list as shown below. Click “OK” and you are done. Now, when you add or delete the listing in column A, the data validation list will get updated automatically.