Which command line switch would be used in NMAP to perform operating system detection?

-OS is not a valid Nmap command-line switch and will cause an error or be ignored by the Nmap engine.

-sO performs an IP Protocol Scan to discover which IP protocols are active on a host, not OS fingerprinting.

-sP performs a ping scan to identify live hosts on a subnet, not to detect or fingerprint operating systems.

The -O flag in Nmap activates OS detection by sending a series of specially crafted TCP and UDP probes to open and closed ports and analyzing responses against a fingerprint database. The TCP/IP stack behavior differences between operating systems allow Nmap to identify the OS type and version with reasonable accuracy. This information is critical during reconnaissance for selecting appropriate exploits or attack vectors.