Answer:
b. Force majeure
Step-by-step explanation:
Force majeure simply refers to unavoidable accident, unexpected incidents, or chance occurrence that impede the fulfillment of a contract by a party it.
Force majeure is a common clause that is usually included in contracts in order to free both parties to the contract from liability or obligation that may occur due the occurrence of unforeseen circumstance beyond their control.
Examples of these unforeseen incidents include a new government regulation or law, riot, war, or an act of God like earthquake, flood, hurricane, and among others.
From the question, Anker is relying on force majeure because it had sent a letter to Allegheny in 2006 claiming physical difficulties at the mine and a change in the enforcement of regulations relating to coal mining near gas wells hampered their ability to extract coal from the mine.