When dealing with Large Data Volumes (LDV)—specifically in the range of "tens of millions of records"—standard import tools and standard APIs are insufficient due to governor limits and processing speeds. For a time-sensitive weekend migration, the Bulk API is the only architecturally sound recommendation.

Why Bulk API is Required:

Parallel Processing: Unlike the SOAP API (Option B), which processes records synchronously (one by one or in small serial batches), the Bulk API is designed for asynchronous processing. It breaks the data into large chunks (up to 10,000 records per batch) and processes them in parallel on the Salesforce application servers.

Minimized API Calls: Tens of millions of records would quickly exhaust an organization's daily SOAP API limits. The Bulk API is optimized to handle massive datasets with significantly fewer API calls.

Weekend Constraint: The "weekend" requirement implies a need for high throughput. The Bulk API is the fastest method available for moving data into Salesforce, making it possible to complete a multi-million record migration within a 48-hour window.

Why other options fail:

NPSP Data Import (Option C): While powerful for processing complex logic (like creating Accounts and Contacts at once), it is built on top of standard Apex processing and is significantly slower than the Bulk API for pure volume.

Data Import Wizard (Option D): This tool is limited to 50,000 records per session, making it impossible to use for a migration of this scale.

A consultant would typically use a tool like Data Loader (configured for Bulk mode) or a dedicated ETL tool (like Informatica or Mulesoft) that utilizes the Bulk API to achieve the necessary performance.