The DevOps team is using Lambda@Edge to perform the A/B testing strategy for a Web application. By Lambda@Edge, logic can be added in the CloudFront CDN in order to choose which content to deliver without the need to touch the application code. Two versions of application contents can be easily tested and analyzed. Lambda functions can change CloudFront contents at below points:

Answer options:

A.CloudFront cache 
B.Viewer request
C.Origin request
D.Origin response
E.Viewer response

Correct Answer – B, C and D
Refer to
https://docs.aws.amazon.com/lambda/latest/dg/lambda-edge.html
https://medium.com/buildit/a-b-testing-on-aws-cloudfront-with-lambda-edge-a22dd82e9d12
for how Lambda works with CloudFront Lambda@Edge:
Option A is incorrect: Because CloudFront cache cannot be configured with a Lambda.
Option B, C and D are CORRECT: In this case, Lambda should be configured in Viewer request or Origin request or Origin Response to choose which version of application to route to.
Viewer Request : The function executes when CloudFront receives a request from a viewer, before it checks to see whether the requested object is in the edge cache.
Origin Request: The function executes only when CloudFront forwards a request to your origin. When the requested object is in the edge cache, the function doesn`t execute.
Origin Response : The function executes after CloudFront receives a response from the origin and before it caches the object in the response. Note that the function executes even if an error is returned from the origin.
The function doesn`t execute in the following cases:
 • When the requested file is in the edge cache
 • When the response is generated from a function that was triggered by an origin request event
Refer page 298 on the below link
https://docs.aws.amazon.com/AmazonCloudFront/latest/DeveloperGuide/AmazonCloudFront_DevGuide.pdf
Option E is incorrect: Viewer response is the response which CloudFront forwards to the viewer. They are not the points to decide which version to send the request to.