What is the default storage format for Fabric's OneLake?

• A. JSON
• B. Delta
• C. CSV
• D. Parquet

Answer: (B)

OneLake defaults to Delta Lake as the storage format because it provides reliable transactional guarantees and governance for data lake tables. Delta Lake adds an ACID layer on top of the data, using a transaction log to track changes, which enables atomic writes, upserts, deletes, and time travel while ensuring consistent reads across users and tools. This reliability and manageability across Fabric analytics workloads is what makes Delta the preferred default. While Parquet is the efficient file format underlying Delta’s data, JSON and CSV are plain formats without transactional semantics, and Delta’s transaction log delivers the governance and robustness that OneLake aims to provide.