Explanation:

All three VMs are in VNET2. Auto registration is enabled for private Azure DNS zone named contoso.com, which is linked to VNET2. So, VM1, VM2 and VM3 will auto-register their host records to contoso.com.
None of the VM will auto-register to the public Azure DNS zone named adatum.com. You cannot register private IPs on the internet (adatum.com) Box 1: Yes Auto registration is enabled for private Azure DNS zone named contoso.com.
Box 2: Yes
Auto registration is enabled for private Azure DNS zone named contoso.com.
Box 3: No
None of the VM will auto-register to the public Azure DNS zone named adatum.com Reference:
https://docs.microsoft.com/en-us/azure/virtual-network/virtual-networks-name-resolution-for-vms-and-role- instances
https://docs.microsoft.com/en-us/azure/dns/private-dns-autoregistration
https://docs.microsoft.com/en-us/azure/dns/private-dns-virtual-network-links