Final answer:
To cancel an incorrect rental car provider selection on the Review Reservation Selections screen, you should click on 'Cancel Reservation'. The correct option is A.
Step-by-step explanation:
If you're on the Review Reservation Selections screen and realize that you have selected the wrong rental car provider, you will need to choose an option that allows you to cancel or change your existing reservation. Among the options listed, the one that directly suggests that you can undo the reservation is Cancel Reservation. Change Reservation or Modify Reservation may allow you to select a different provider without cancelling, but if you are certain you want to cancel the original selection, Cancel Reservation is the most appropriate link to select.
This will likely take you to a confirmation screen where you can confirm the cancellation of your booking. Once the reservation is cancelled, you can then proceed to make a new selection with the correct rental car provider. 'Delete Reservation' might also cancel the reservation, but 'Cancel Reservation' typically is the standard terminology used in booking systems to refer to the process of cancelling a reservation.