Answer - D.
This is also mentioned in the AWS Documentation.
FIFO (First-In-First-Out) queues are designed to enhance messaging between applications when the order of operations and events is critical or where duplicates can't be tolerated, for example:
Ensure that user-entered commands are executed in the right order.
Display the correct product price by sending price modifications in the right order.
Prevent a student from enrolling in a course before registering for an account.
Options A and B are incorrect since an existing Standard queue can't be changed to FIFO.