The correct answer is Azure Cosmos DB.

Azure Cosmos DB is a globally distributed, multi model database that offers guaranteed single digit millisecond read and write latency at the 99th percentile and automatic horizontal partitioning for high throughput. It also provides automatic indexing and tunable consistency levels which make it a good fit for live chat workloads that need extremely fast reads and writes together with durable storage and optional global replication.

Azure Cache for Redis provides very low latency because it is an in memory cache, but it is primarily a cache rather than a durable primary message store. Persistence options are limited and using it as the sole storage layer risks data loss or added complexity for durability and global replication.

Azure Blob Storage is object storage that is optimized for large binary objects and high throughput streaming. It is not designed for the small, low latency indexed reads and writes and the rich querying patterns that a chat message store typically requires.

Azure SQL Database is a managed relational database that supports transactions and complex queries. It can scale well for many scenarios but it does not natively provide the same predictable ultra low latency and global distribution at massive scale without additional sharding or architectural complexity.

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