If a property is under contract to a buyer, another buyer may use the __________________ to enter into a contract if the first contract fails to close.

The correct answer is the Addendum for Back-up Contract. This addendum allows a second buyer to have a chance to purchase the property if the first buyer's contract fails to close. Essentially, it creates a secondary position for the second buyer, ensuring they can step in and make the purchase under the agreed-upon terms should the original transaction not proceed.

This is particularly important in a real estate context where properties in demand can have multiple interested buyers. By utilizing the Addendum for Back-up Contract, the seller retains the opportunity to sell the property without needing to relist if the first contract falls through.

The other options represent different scenarios or documents that do not fit the situation of securing a potential purchase opportunity should the first contract fail. For instance, the Addendum for Sale of Other Property by Buyer addresses situations where the buyer needs to sell another property before completing the purchase, while the 1-4 Family Residential Contract is simply a purchase agreement, not designed for back-up positions. The Notice of Buyer's Termination of Contract deals with the process of formally ending a contract, which does not help with creating a new opportunity for another buyer.