I've noticed there are a lot of questions about decoupling services and SQS is almost always the answer.

D SNS fan out

They don't look like real answers from the official exam...

Each option is badly worded: A: "generate the thumbnail and alert the user" doesn't sound sequential so could alert the user during, before or after the thumbnail generation whichever way you interpret it. B: this is sequential and won't alert until the steps are complete D: Could work without with the risk of notification loss so C is better but this is also ok

Safe answer is C but B is so badly worded that it can mean anything to confuse people. Step functions to use tiers. What if on of the step is to inform to the user and move on to next step. Anyway, I'll chose C for the exam as it is cleaner.

... asynchronously dispatch ... => Amazon SQS

Asynchronous, Decoupling = Amazon Simple Queue Service

SQS is a fully managed message queuing service that can be used to decouple different parts of an application.

Answers B and D alert the user when thumbnail generation is complete. Answer C alerts the user through an application message that the image was received.

B: Use cases for Step Functions vary widely, from orchestrating serverless microservices, to building data-processing pipelines, to defining a security-incident response. As mentioned above, Step Functions may be used for synchronous and asynchronous business processes.

why not B?

Creating an Amazon Simple Queue Service (SQS) message queue and placing messages on the queue for thumbnail generation can help separate the image upload and thumbnail generation processes.

C The key here is "a faster response time to its users to notify them that the original image was received." i.e user needs to be notified when image was received and not after thumbnail was created.

A looks like the best way , but its essentially replacing the mentioned app , that's not the ask

Selected Answer: A https://docs.aws.amazon.com/lambda/latest/dg/with-s3-tutorial.html

C is the only one that makes sense

Use a custom AWS Lambda function to generate the thumbnail and alert the user. Lambda functions are well-suited for short-lived, stateless operations like generating thumbnails, and they can be triggered by various events, including image uploads. By using Lambda, the application can quickly confirm that the image was uploaded successfully and then asynchronously generate the thumbnail. When the thumbnail is generated, the Lambda function can send a message to the user to confirm that the thumbnail is ready. C proposes to use an Amazon Simple Queue Service (Amazon SQS) message queue to process image uploads and generate thumbnails. SQS can help decouple the image upload process from the thumbnail generation process, which is helpful for asynchronous processing. However, it may not be the most suitable option for quickly alerting the user that the image was received, as the user may have to wait until the thumbnail is generated before receiving a notification.