Which of the following is a file server that provides data access to multiple, heterogeneous machines/users on the network?

A Storage Area Network (SAN) provides block-level storage, not file-level, and typically connects directly to servers rather than serving multiple heterogeneous clients directly over a general network.

Network-attached storage (NAS) is a specialized server that provides file-level data storage to a network of heterogeneous clients. It connects directly to the network and allows multiple users and machines to access shared files using standard network file-sharing protocols like NFS or SMB/CIFS.

A Hardware Security Module (HSM) is a physical computing device that safeguards and manages digital keys and performs cryptographic operations, not a file server.

A Content Delivery Network (CDN) is a geographically distributed network of proxy servers and their data centers, providing high-availability and high-performance access to web content, not general file sharing among network clients.