Final answer:
The best option to update 50,000 quotes on a BigMachines site with a new line item attribute would be to perform a mass update on all quotes, as it is the most efficient and least error-prone method compared to manual updates or individual function creation.
Step-by-step explanation:
To update 50,000 quotes with a new line item attribute in a BigMachines site, the most efficient method would be to run a mass update on all quotes. Updating the quotes manually would be time-consuming and prone to errors. Option C, stating that existing line items will update automatically, is unlikely, because typically in data management systems, existing records don't update without a prompt. Lastly, while creating a new Commerce library function could work, it would require significant effort to pass each quote number as input, making the mass update method more sensible.
In practice, to perform a mass update, you would typically use the site's built-in tools or advanced features. This may involve exporting the quotes, applying changes in a batch, and then re-importing them. The specific procedure may vary based on the system's capabilities and the data format, so it's essential to consult the BigMachines documentation or support team for the recommended workflow.