The correct answer is D. Using an AWS Glue extract, transform, and load (ETL) job to convert the .csv files to Parquet format leverages Glue's built-in capabilities for data transformation, which is both scalable and efficient. By triggering this ETL job with an AWS Lambda function upon each S3 PUT event, the process automates the file conversion with minimal operational overhead. This approach handles large data volumes effectively and is more suitable for handling the size and frequency of .csv files described in the scenario compared to other options.