The correct answer is adding a public DNS record . Microsoft documents that when you add a custom domain to Microsoft 365, you must first prove ownership of that domain before Microsoft 365 can use it for services such as Exchange Online, SharePoint, and user sign-ins. The standard verification method described by Microsoft is to add a DNS record at your domain registrar or DNS hosting provider. Microsoft commonly uses a TXT record for verification, although in some cases an MX record can also be used depending on the setup flow. This is why “adding a public DNS record” is the right completion for the sentence.

The other choices are not the standard Microsoft 365 domain verification process. Microsoft’s admin guidance does not use confirming your business address , uploading a certificate , or uploading a webpage as the normal method for proving ownership of a domain in Microsoft 365. Domain verification is specifically tied to DNS because DNS is the authoritative public system used to prove control over the domain name.