A large Tier-1 telco with 20 million subscribers needs to move all of their customer...

For a Tier-1 Telco with a massive data volume (20 million subscribers), a "Big Bang" migration (shutting down and moving everything at once) is impossible due to the required downtime and risk. The Consultant must recommend a Hybrid Migration Strategy that ensures business continuity.

Staged/Partitioned Migration (The "Bulk" Strategy):

To handle the 20 million records, the data must be partitioned into logical blocks (e.g., by Region, Billing Cycle, or Account Status).

These blocks are migrated in multiple stages (batches) over time in the background. This minimizes system load and allows for validation at each stage.

This aligns with the prompt's requirement to address the "long time to migrate."

On-Demand / Just-In-Time (JIT) Migration (The "Continuity" Strategy):

The critical requirement is to process orders "uninterrupted through the Salesforce platform".

If a customer whose data has not yet been migrated calls to place an order, the system cannot tell them to wait.

Instead, the system must perform an On-Demand (Just-In-Time) migration for that specific customer record immediately, bringing their asset data into Salesforce so the order can be captured and processed within Salesforce.

Why other options are incorrect:

A (Disable production): Impossible for a Tier-1 Telco; downtime for 20M records would be unacceptable.

B (Direct requests to legacy): This violates the requirement to process orders through the Salesforce platform. Relying on the legacy system for active orders creates a "split brain" scenario and prevents Salesforce from becoming the system of record.

C (Migrate on-demand...): While partially correct, Option D is the better architectural answer because it explicitly defines the Partitioning/Staging strategy required to manage the bulk volume, whereas C is less specific about how the background migration is handled.