Final answer:
A license key is a unique code that registers and activates software, distinguishing it from passwords, usernames, and security codes used for different purposes.
Step-by-step explanation:
The unique identifier that authenticates the use of the software is a license key. A license key is a code provided by the software vendor after purchase that registers and activates the software to make it eligible for updates and support. Other options like a password, a username, and a security code serve different purposes.
A password and a username are generally used for accessing accounts, while a security code can be used for transaction verification or account recovery.