Box 1: No -
Azure DNS provides automatic registration of virtual machines from a single virtual network that's linked to a private zone as a registration virtual network. VM5 does not belong to the registration virtual network though.
Box 2: No -
Forward DNS resolution is supported across virtual networks that are linked to the private zone as resolution virtual networks. VM5 does belong to a resolution virtual network.
Box 3: Yes -
VM6 belongs to registration virtual network, and an A (Host) record exists for VM9 in the DNS zone.
By default, registration virtual networks also act as resolution virtual networks, in the sense that DNS resolution against the zone works from any of the virtual machines within the registration virtual network.
Reference:
https://docs.microsoft.com/en-us/azure/dns/private-dns-overview
