You’ve been given the requirement to customize the content which is distributed to users via a Cloudfront Distribution. The content origin is an S3 bucket. How could you achieve this?

Answer options:

A.Add an event to the S3 bucket. Make the event invoke a Lambda function that would customize the content.
B.Add a Step Function. Add a step with a Lambda function just before the content gets delivered to the users.
C.Consider using Lambda@Edge.
D.Consider using a separate application on an EC2 Instance for this purpose.

Answer – C
The AWS Documentation mentions the following.
Lambda@Edge is an extension of AWS Lambda, a compute service that lets you execute functions that customize the content that CloudFront delivers. You can author functions in one region and execute them in AWS locations globally closer to the viewer, without provisioning or managing servers. Lambda@Edge scales automatically, from a few requests per day to thousands per second. Processing requests at AWS locations closer to the viewer than on origin servers significantly reduces latency and improves the user experience.
Option A is incorrect because although adding an event to the S3 bucket and then invoking the event by the Lambda function to customize the content may do the needful, a lot of manual intervention is required. With Lambda @ Edge, it can be done more easily.
Option B is incorrect because Step Function is normally used to sequence AWS Lambda functions and multiple AWS services. This scenario is not suitable.
Option D is incorrect because adding EC2 is not cost-efficient. 
For more information on Lambda@Edge, please refer to the below Link-
https://docs.aws.amazon.com/AmazonCloudFront/latest/DeveloperGuide/lambda-at-the-edge.html