For personal or business use where reliability and compliance matter, only purchase from Microsoft or a clearly authorized retailer. For low-risk personal experimentation, a “verified” key under $50 may work temporarily, but you should expect zero recourse if deactivated.
Microsoft Office 2021 Home & Business for PC is a one-time purchase
Verified Guide: Microsoft Office 2021 Home & Business for PC
During activation, you should always be directed to ://office.com . If a site asks you to download a strange third-party "activator," it is not verified.