The best way to create a list that contains all prospects who complete the Contact Us form, but will later remove any prospects if they become marked as opted out, is to use a dynamic list. A dynamic list is a list that automatically adds or removes prospects based on criteria that you define. In this case, the criteria would be based on the form completion and the opt out status of the prospect. A dynamic list would also add prospects back to the list if they opt back in. The other options are not as efficient or effective as a dynamic list, as they would require manual intervention or additional automation steps