Final answer:
A blank check is the least effective method of acknowledging an order because it lacks important details. A blank check does not provide any specific information about the order and lacks the necessary details for proper acknowledgment.
Step-by-step explanation:
The least effective method of acknowledging an order is a blank check. A blank check does not provide any specific information about the order and lacks the necessary details for proper acknowledgment. On the other hand, a postcard, an acknowledgement form, or a duplicate invoice all contain important information like order number, item details, customer details, and an acknowledgment statement.