Explanation:

In Microsoft Azure, connectivity between virtual networks depends on IP address space, region, and subscription constraints.
1## Site-to-Site VPN between VNET1 and VNET2 - YES:
Azure supports a Site-to-Site VPN connection between two VNets in different regions or subscriptions as long as both have non-overlapping address spaces and contain a VPN gateway. In this case, VNET1 (10.10.10.0/24 - West Europe) and VNET2 (172.16.0.0/16 - West US) have unique address spaces, so a S2S connection is possible.
2## VNET1 and VNET2 Peering - YES:
VNet peering enables direct communication over the Microsoft backbone network with low latency and no gateway requirement. Global VNet Peering supports VNets in different Azure regions (e.g., West Europe and West US) as long as address spaces do not overlap-which is true here.
3## VNET1 and VNETA Peering - NO:
VNETA's address space (10.10.128.0/17) overlaps with VNET1 (10.10.10.0/24), because both belong to the same 10.10.0.0/8 range. Azure explicitly blocks VNet peering between VNets with overlapping address spaces to avoid routing conflicts.
According to Microsoft Learn ("Virtual network peering - requirements and constraints"), peering is only supported when address spaces do not overlap. Therefore, VNET1 and VNETA cannot be peered.