What is the MOST secure way to grant the Lambda function access to the S3 bucket and the DynamoDB table?

Explanations:

Attaching the existing IAM policy directly to the Lambda function is not the best practice. It does not provide a separate role with the principle of least privilege and makes it harder to manage permissions and audit access.

Creating an IAM role for the Lambda function and attaching the existing IAM policy to the role is the most secure approach. It allows for fine-grained permission management, adheres to the principle of least privilege, and can be easily modified without changing the Lambda function’s code or configuration.

Creating an IAM user and using its access keys in environment variables is insecure. This method exposes the credentials within the Lambda function, increasing the risk of credential leakage and does not leverage the temporary security credentials that IAM roles provide.

Adding the AWS account root user access key ID and secret access key as environment variables is highly insecure and a bad practice. It poses a significant security risk, as these credentials would have full access to the AWS account and could be easily compromised.