Generally speaking, command is for application-level or os-level commands, control is for text edition, and alt is for alternate characters (all can be shifted and command "overrides" the rest).
You're right, it's Option-u. Most of the key labels on my MacBook have long since been scratched away.
This has happened with every single Apple keyboard I've ever used. I suspect it's my fault, since I'm a key pounder, having learned to type on an IBM Selectric typewriter.
option-u (aka alt-u).
Generally speaking, command is for application-level or os-level commands, control is for text edition, and alt is for alternate characters (all can be shifted and command "overrides" the rest).