Choice "C" is correct. Failure to give adequate assurances when reasonably demanded is a form of anticipatory repudiation. It constitutes a breach and discharges the buyer.
Choice "b" is incorrect. If the goods are destroyed after the risk of loss passes to the buyer, the buyer is obligated to pay for the goods since the buyer had the risk.
Choice "d" is incorrect. If the delivery terms are impracticable, the buyer must accept delivery by some other reasonable means.
Choice "a" is incorrect. If the buyer repudiates, the seller has the right to hold the buyer to the contract; the buyer is not discharged unless the seller cancels. If the buyer retracts the repudiation in a timely fashion, the contract is treated as if there were no repudiation and the buyer remains liable in full.