D. Leveraging a controlled feed load with 10 batches per job.

This recommendation is based on the following considerations:

Using the Bulk API in serial mode processes batches one at a time, which is crucial for avoiding record locking issues, especially when dealing with large volumes of data. This is particularly important when multiple records could be related or linked to the same Salesforce records (e.g., orders related to specific accounts).

A controlled feed with a specific number of batches per job (in this case, 10) helps in managing the load effectively. It ensures that the system is not overwhelmed with too much data at once, which can lead to performance issues. This approach allows for more manageable and predictable data loading processes, which is essential when dealing with millions of records.