Please, could you explain why the C option is the correct answer of that question.

[Mitko4erto]

You plan to move a distributed on-premises app named App1 to an Azure subscription.
After the planned move, App1 will be hosted on several Azure virtual machines.
You need to ensure that App1 always runs on at least eight virtual machines during planned Azure maintenance.
What should you create?

• A. one virtual machine scale set that has 10 virtual machines instances
• B. one Availability Set that has three fault domains and one update domain
• C. one Availability Set that has 10 update domains and one fault domain
• D. one virtual machine scale set that has 12 virtual machines instances

Answer : C

[#2]

@Mitko4erto 

In a planned maintenance  when you have 5 update domains 4  are accessible while the 5th is updated and rebooted . So if you have 10 UD 2  vms will be rebooted while 8 will be accessible .

0 1 2 3 4 5   --> 5 is  Off

0 1 2 3 4 5   --> 5 is  Off