Which of the following is used for remote file access by UNIX/Linux systems?

NetWare Core Protocol (NCP) is proprietary to Novell NetWare environments and is not used by UNIX/Linux systems for remote file access.

Common Internet File System (CIFS) is a dialect of SMB developed by Microsoft and is primarily associated with Windows file sharing, not UNIX/Linux native access.

Server Message Block (SMB) is a Windows-centric protocol; while Samba implements SMB on Linux for interoperability with Windows, SMB itself is not the native remote file access method for UNIX/Linux systems.

Network File System (NFS) was developed by Sun Microsystems and is the standard protocol for remote file access in UNIX and Linux systems. It allows a client to mount remote directories over a network and interact with them as if they were local. It operates at the application layer and is deeply integrated into UNIX/Linux networking stacks.