Millions of Accounts are updated every quarter from an external system.

Comprehensive and Detailed Explanation:

When dealing with Large Data Volumes (LDV)—specifically in the range of hundreds of thousands to millions of records—the Bulk API (Option C) is the only optimal choice.

The Bulk API is based on REST principles but is optimized for processing large sets of data asynchronously. Instead of processing records one by one or in small synchronous chunks (which would hit time limits and consume many API calls), the Bulk API allows you to upload a CSV file or JSON batch. Salesforce then processes these records in the background using parallel processing.

Options A and B (REST/SOAP) are designed for real-time, synchronous interactions with a limited number of records and would hit rate limits or timeout for millions of records. Option D is for custom logic and is subject to the same synchronous execution limits as standard REST.