Explanation:
The total regional vCPUs is 20 so that means a maximum total of 20 vCPUs across all the different VM sizes.
The deallocated VM with 16 vCPUs counts towards the total. VM20 and VM1 are using 18 of the maximum 20 vCPUs leaving only two vCPUs available.
Reference:
https://docs.microsoft.com/en-us/azure/virtual-machines/windows/quotas

Explanation:
They are all Global admins so they can all modify user permission. i.e add self as owner etc.
You can be GA in one of the subscription, it doesn't mean that you can create the resources in all subscription. As a Global Administrator in Azure Active Directory (Azure AD), you might not have access to all subscriptions and management groups in your directory. Azure AD and Azure resources are secured independently from one another. That is, Azure AD role assignments do not grant access to Azure resources, and Azure role assignments do not grant access to Azure AD.
However, if you are a Global Administrator in Azure AD, you can assign yourself access to all Azure subscriptions and management groups in your directory
Reference:
https://docs.microsoft.com/en-gb/azure/role-based-access-control/elevate-access-global-admin

Statement 1: Yes
All client computers in the Paris office will be joined to an Azure AD domain.
A virtual network named Paris-VNet that will contain two subnets named Subnet1 and Subnet2.
Microsoft Windows Server Active Directory domains, can resolve DNS names between virtual networks. Automatic registration of virtual machines from a virtual network that's linked to a private zone with auto-registration enabled. Forward DNS resolution is supported across virtual networks that are linked to the private zone.
Statement 2: Yes
A virtual network named ClientResources-VNet that will contain one subnet named ClientSubnet You plan to create a private DNS zone named humongousinsurance.local and set the registration network to the ClientResources-VNet virtual network.
As this is a registration network so this will work.
Statement 3: No
Only VMs in the registration network, here the ClientResources-VNet, will be able to register hostname records. Since Subnet4 not connected to Client Resources Network thus not able to register its hostname with humongoinsurance.local Reference:
https://docs.microsoft.com/en-us/azure/dns/private-dns-overview
https://docs.microsoft.com/en-us/azure/virtual-network/virtual-networks-name-resolution-for-vms-and-role-instances

Explanation
Only a global administrator can add users to this tenant.
References:
https://docs.microsoft.com/en-us/azure/devops/organizations/accounts/add-users-to-azure-ad