Which CLI command is used to register a Cisco FirePOWER sensor to Firepower Management Center?

The syntax `configure system add <host><key>` is incorrect for manager registration, as 'system' is not the correct context.

The syntax `configure manager <key> add host` has the key and host arguments in the wrong order relative to the standard command format.

`configure manager delete` is used to remove an existing registration to the FMC, not to add a new one.

The command `configure manager add <host> <key>` is the correct CLI command used on a Cisco FirePOWER sensor to register it with the Firepower Management Center (FMC). `<host>` specifies the IP address or hostname of the FMC, and `<key>` is the registration key generated on the FMC.