The solution requires multiple writable replicas across different regions. The Strong consistency level is not available for Azure Cosmos DB accounts configured with multi-region writes. Therefore, the strongest possible consistency level that meets this core requirement is Bounded Staleness. Bounded Staleness guarantees that reads lag behind writes by a configurable time interval or a specific number of versions, providing predictable consistency. It is the strongest consistency model that supports both multi-region writes and provides a latency-based Service Level Agreement (SLA) for write operations, fulfilling all the solution's requirements.

B. strong: This consistency level is not available for accounts configured with multiple write regions, which is a mandatory requirement for the solution.

C. session: While it supports multi-region writes, Session consistency is weaker than Bounded Staleness. The question explicitly asks for the strongest level.

D. consistent prefix: This is a weaker consistency level than both Bounded Staleness and Session, and therefore not the strongest available option that meets the criteria.

1. Microsoft Docs: Consistency levels in Azure Cosmos DB.

Section: "Consistency levels and multiple write regions"

Content: "If your Azure Cosmos DB account is configured with multiple write regions

you can't choose strong consistency. This level is only available for accounts operating in a single write region." This directly invalidates the 'strong' option.

Section: "Consistency levels"

Content: The document lists the consistency levels in order of strength from strongest to weakest: Strong

Bounded Staleness

Session

Consistent Prefix

and Eventual. This confirms Bounded Staleness is the strongest level available after Strong.

2. Microsoft Docs: Availability and performance SLAs for Azure Cosmos DB.

Section: "Latency"

Content: This document confirms that Azure Cosmos DB provides a latency SLA for writes for Bounded Staleness

Session

Consistent Prefix

and Eventual consistency levels. The Strong consistency level prioritizes its Recovery Time Objective (RTO) of zero over a latency SLA for writes

which further supports Bounded Staleness as the correct choice given the requirement for a write latency SLA.

3. Microsoft Docs: Configure your Azure Cosmos DB account with multiple write regions.

Section: "Consistency levels"

Content: "When you configure your account for multiple write regions

the strong consistency level is not available. The default consistency level for a multi-region account is session consistency. However

you can relax the consistency to get higher performance." This reinforces that Strong is not an option and that other levels

including the stronger Bounded Staleness

are available.