The correct answer is Azure Cosmos DB. It is the service that best fits a requirement to hold JSON records without a fixed schema while providing flexible indexing and querying capabilities.

Azure Cosmos DB is a globally distributed, multi model database that natively stores JSON documents and operates as a schema free document store. It automatically indexes properties by default so you can run rich, SQL like queries over JSON without needing to predefine secondary indexes. It also provides multiple APIs and consistency options along with predictable low latency and comprehensive SLAs, which makes it well suited to flexible document workloads that require powerful query capabilities.

Azure Blob Storage is object storage for large unstructured files and blobs. You can store JSON files in blobs but it does not provide automatic indexing or rich, ad hoc querying of JSON content without pairing it with additional services.

Azure SQL Database is a relational database that requires defined schemas for most workloads. It does include JSON functions for storing and querying JSON, but it is not optimized for schema free document workloads or for automatic indexing of arbitrary JSON properties in the same way a document database is.

Azure Table Storage is a simple key value NoSQL table store with limited querying capabilities. It does not offer the flexible document indexing or the rich query surface that you get from a document database such as Cosmos DB.

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