HOTSPOT
You have an Office 365 subscription that has a site collection with a URL of https://contoso.sharepoint.com.
Users report that they are prevented from uploading documents to the site because of free space issue. You
increase the storage quota for the site collection to 1000 MB.
You need to ensure that you receive a warning message when the site collection uses 90 percent of the
available space.What command should you run? To answer, select the appropriate options in the answer area.
NOTE: Each correct selection is worth one point.
Hot Area:
Answer: 
This is correct:
https://technet.microsoft.com/en-us/library/fp161394.aspx
Set-SPOSite -identity https://contoso.sharepoint.com -StorageQuotaWarningLevel 90
-StorageQuotaWarningLevel argument is percentage of Storage Quota
@leonardo, it is not true. The -StorageQuotaWarningLevel argument is expressed in MB
https://docs.microsoft.com/en-us/powershell/module/sharepoint-online/set-sposite?view=sharepoint-ps