You are evaluating the name resolution for the virtual machines after the planned implementation of...

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