Option A is the correct answer because it leverages AWS IAM authentication (AWS_IAM auth type) for Lambda function URLs, ensuring only authenticated and authorized IAM entities in the QA IAM group can invoke the Lambda functions. The approach involves creating a script to loop through the Lambda functions to add URLs with AWS_IAM auth type and another script to create an IAM identity-based policy that grants the lambda:InvokeFunctionUrl action. This policy is then attached to the QA IAM group, allowing controlled and secure access according to AWS best practices.