The correct answer is B. To ensure that the orders are processed in the order that they are received, an Amazon SQS FIFO (First-In-First-Out) queue should be used. A FIFO queue guarantees that messages are processed exactly in the same sequence in which they are sent. By configuring the SQS FIFO queue to invoke an AWS Lambda function, the system ensures that each order is processed in the correct order. The other options do not guarantee order processing: SNS does not ensure ordering (option A), blocking requests using an API Gateway authorizer is not a practical or scalable solution (option C), and an SQS standard queue does not guarantee ordering (option D).