Which feature allows you to upload a CSV file containing item data to combine with Analytics data?

The feature that enables you to upload a CSV file containing item data to integrate with your Analytics data is known as Data import. This functionality allows users to enrich their existing data in Google Analytics by uploading additional data, such as information about products or other relevant attributes that can enhance analysis and reporting. With Data import, you can combine offline data with online data for deeper insights and more comprehensive reporting.

The other options serve different purposes. Data Studio is primarily a reporting and visualization tool that connects with various data sources, but it does not facilitate the import of data directly into Analytics. The Measurement Protocol is a method for collecting data from any web-connected device and sending it to Google Analytics, but it does not involve uploading CSV files. Custom Dimensions are used to collect additional information about users or sessions that is not automatically tracked by Analytics; they do not integrate CSV file data either. Thus, the correct answer is Data import, as it directly relates to the capability of uploading CSV files for use within Google Analytics.