Answer:
this promise is enforceable only if it is agreed upon in writing
Step-by-step explanation:
In the scenario described in the question it can be said that this promise is enforceable only if it is agreed upon in writing. This is because by putting it in writing all details of the contract are displayed for both parties to read/analyze and decide whether they actually want to agree to this agreement/contract or not. Once the contract is signed and agreed upon by both parties it can then be completely enforced because both parties knew exactly what they were getting into at the time of signing.