Azure Blob Storage with the hierarchical namespace enabled. It organizes data into real directories, allows atomic folder operations, and supports POSIX-compliant access control lists (ACLs).
In DP-900 it is the typical foundation of a data lake and integrates with Databricks, Synapse, and Fabric. To create it, you enable the hierarchical namespace on a storage account.
Azure Data Lake Storage Gen2 is a concept that can show up on the exam. Check the full guide to see it in context.