You are a lead developer for an application that uses WebSockets through API Gateway to push payloads between the clients and server. Your API has a proxy integration with Lambda. When the client connects for the first time, it receives a preflight error message. Which steps will you take to resolve this issue?

Answer options:

A.Enable CORS using the API Gateway console.
B.Setup the OPTIONS method and set up the required OPTIONS response headers in API Gateway.
C.Make changes to your backend to return “Access-Control-Allow-Headers” and “Access-Control-Allow-Origin” headers.
D.A and
B.E.A, B and
C.

Answer: E
Option A is incorrect because using the Management Console to enable CORS will by itself not resolve the issue.
Option B is incorrect because when using a proxy integration with Lambda, it is necessary to add “Access-Control-Allow-Headers” and “Access-Control-Allow-Origin” headers to the response in the Lambda function as a proxy integration will not return an integration response.
Option C is partially correct as it on its own it will not resolve the issue as laid in the AWS documentation.
Option D is partially correct as it on its own it will not resolve the issue as laid in the AWS documentation.
Option E is CORRECT. When using a proxy integration with Lambda, it is necessary to add “Access-Control-Allow-Headers” and “Access-Control-Allow-Origin” headers the response in the Lambda function as a proxy integration will not return an integration response. All three steps need to be used when enabling CORS for an API with a Lambda proxy integration.
Reference:
https://docs.aws.amazon.com/apigateway/latest/developerguide/how-to-cors.html