The correct answer is C. Adding a resource policy to the production Lambda function to allow only the production S3 bucket to invoke the function ensures that only events from the designated production S3 bucket can trigger the production Lambda function. This prevents development or other non-production buckets from inadvertently invoking production Lambda functions, thereby maintaining environment integrity and adhering to security best practices. While option B also mentions following best practices by separating environments into different accounts, it is not strictly necessary if the resource policy can effectively isolate the environments within the same account.