The Option Key (along with Control and Command) will be on keyboards made by Apple and some 3rd party keyboards made specifically for Macs. Other USB keyboards will work but may have Windows-specific markings.
The key layouts from left to right on Mac keyboards are (bottom row):
Control - Option - Command - Space Bar - Command - Option - Control
If you are using a Windows keyboard, the layout is the same but they keys could be marked differently. In other words, the leftmost key will be the Control key when booted into MacOS regardless of what is marked on it for when you are booted into Windows OS.
Some compact keyboards might eliminate the left or right keys, but there will always be at least one set. For example it could be:
Control - Option - Command - Space Bar -
There are also symbols used to describe these keys in support documents.
^ is Control
A stairstep looking symbol is Option
A cloverleaf looking symbol is Command
Some people will refer you to the "Apple Key". There is no such thing but it is common for the Command key to be referred to this way as on many Apple keyboards the Apple Character is on that key. It is usually not on 3rd party keyboards, which is why you should always refer to it as the Command key, since referring to it as the Apple key will confuse many users. Apple never refers to this as the "Apple Key".
Alternative name for Option key is Alt, but again Apple never refers to it as Alt; it is there for Windows/UNIX compatibility.