Answer:
A. The rate when the inventory was paid for
Explanation:
The U.S. company should register the inventory purchase in their balance sheet using the $/C$ exchange rate at that date the inventory was paid for since that would represent the actual monetary value spent on inventory. The rate is subject to change and, therefore, using the exchange rate at the time of delivery, sale or at the balance sheet date, could incorrectly represent the company's inventory expenses.