Which of the following can be used when installing the OS without a boot disk?

ZFS (Zettabyte File System) is a combined file system and logical volume manager, not a method for network booting an OS.

VMFS (Virtual Machine File System) is a clustered file system developed by VMware for its ESXi servers to store virtual machine files, not for network booting a physical OS.

PXE (Preboot eXecution Environment) allows a computer to boot from a network interface, fetching boot files and an operating system image over the network, thereby eliminating the need for a local boot disk or DVD. This method is commonly used for large-scale OS deployments.

PBX (Private Branch Exchange) is a private telephone network used within a company, entirely unrelated to OS installation.