Final answer:
The statement that a message becomes visible again if 'DeleteMessage' is not called before the visibility timeout expires is true. So option (a) is correct.
Step-by-step explanation:
If your system doesn't call DeleteMessage for that message before the visibility timeout expires, the message again becomes visible is True. This concept is an integral part of message queue services, such as Amazon SQS (Simple Queue Service).
In such services, when a message is received, it becomes invisible to other consumers, preventing them from processing the same message concurrently. The visibility timeout is a period during which the message is invisible while a consumer processes it.
If the consumer successfully processes the message, it should call DeleteMessage to remove it from the queue. If this is not done before the visibility timeout expires, the message will become visible again and may be processed by another consumer, potentially leading to the message being processed more than once.