Correct answer - "CloudWatch Event Rules" : You can set up notifications for a repository so that
repository users receive emails about the repository event types you specify. When you configure
notifications, AWS CodeCommit creates an Amazon CloudWatch Events rule for your repository. This
rule responds to the event types you select from the preconfigured options in the AWS CodeCommit
console.
"CodeBuild" - CodeBuild is a code building service and won't help with the comments on pull
requests in CodeCommit
"AWS Lambda function that periodically checks for new comments" - This would work but would be
inefficient
"SQS" - You would consider SQS for message queueing