A. Frustration

Explanation:
A contract is discharged by frustration when unforeseen circumstances make it impossible to carry out the terms of the contract. Frustration occurs when an event beyond the control of either party fundamentally changes the nature of the contractual obligations or makes it legally or physically impossible to fulfill the contract.

Reasoning:
Frustration discharges the parties from their contractual duties because the contract can no longer be performed due to circumstances beyond their control, without any fault on the part of either party.