Final answer:
The question asks for an acceptable excuse for non-performance of a contract, and the correct option is 'Failure of condition'. In contract law, when a condition precedent does not occur, the parties may be excused from performing the contract.
Step-by-step explanation:
The acceptable excuse for delay or non-performance of a contract from the options provided is Failure of condition. In contract law, a failure of condition occurs when a condition precedent, which is a specific event or action that must occur before the performance is required, does not happen. This may release the parties from their contractual obligations.
Strikes and labor troubles could potentially be an excuse under the concept of force majeure, depending on the specific language in the contract. The inability to obtain a permit or license may be an excuse if the contract includes this as a condition. Hardship and inconvenience, however, are typically not acceptable excuses as they do not prevent performance but rather make it more difficult or less profitable.