Final answer:
Both commercial impracticability and frustration of purpose are defenses to a breach of contract claim, used when unforeseen events significantly alter the expected outcomes of the contract; impracticability concerns difficulty in performance, while frustration of purpose concerns the contract's main objective being thwarted.
Step-by-step explanation:
You asked about the difference between commercial impracticability and frustration of purpose as they relate to contracts. They are both doctrines that can be used under the right circumstances in the event that unforeseen events occur that significantly alter the expected outcome of the contract. However, they are used in slightly different conditions and have different implications.
Commercial impracticability occurs when a party finds it extremely difficult or unreasonably expensive to perform its obligations under a contract due to an unforeseen event. This defense doesn't necessarily terminate the contract but can be the basis for a renegotiation of the contract terms. It is identified in the third option: Commercial impracticability and frustration of purpose are both defenses to a breach of contract claim.
On the other hand, frustration of purpose occurs when the entire purpose behind entering the contract has been destroyed by an uncontemplated event, making the performance of the contract pointless. This can be grounds for termination of the contract. This concept is reflected in the third option as well.
Therefore, the correct answer to your question is the third option: Commercial impracticability and frustration of purpose are both defenses to a breach of contract claim.