The correct answer is D. The optimal way to gradually roll out changes to AWS Lambda function code in a production environment is to update the code and create a new version. Then, create a Lambda function alias that can manage traffic splitting between the two versions. The alias allows configuring traffic weights so that 90% of the traffic goes to the current production version and 10% to the new version. This method ensures a controlled and incremental deployment of the new code to minimize potential issues. Options A, B, and C do not provide the same level of control and are not appropriate for the described scenario.