The correct answer is: A. decreasing the vendor lead time.
The vendor lead time is the time it takes for a supplier to deliver an order. Decreasing the vendor lead time will reduce the amount of time that inventory is sitting on the shelf, which will reduce the average inventory level.
Option B, increasing the usage rate, will increase the amount of inventory that is used, which will increase the average inventory level.
Option C, decreasing the order quantity, will reduce the amount of inventory that is ordered at a time, which will reduce the average inventory level.
Option D, lowering the reorder point, will cause orders to be placed more frequently, which will increase the amount of inventory that is on hand, which will increase the average inventory level.
Option E, none of the above, is incorrect.