- HTTP request
- Measurement Protocol
- Modify event
- Data import
The correct answer is: Data import
You can upload CSV files that contain external data to an Analytics property. You can export those CSV files from an offline business tool like your CRM or CMS system, or for smaller amounts of data, you can create the files manually in a text editor or spreadsheet. You can import to Analytics using data import.
When you import data, you create a data source. A data source is the combination of the CSV file you want to upload and a mapping of existing Analytics fields to the fields in your CSV. The CSV file has to include the same fields, or subset of fields, as the original. If you want to import different fields for the same data type, you need to delete the existing data source and create a new one.
Read more here: https://support.google.com/analytics/answer/10071301