Creating Virtual Network Gateways between two Vnets in diffirent Subscriptions

QUESTION 3: You have two subscriptions named Subscription1 and Subscription2. Each subscription is associated to a different Azure AD tenant. Subscription1 contains a virtual network named VNet1. VNet1 contains an Azure virtual machine named VM1 and has an IP address space of 10.0.0.0/16. Subscription2 contains a virtual network named VNet2. Vnet2 contains an Azure virtual machine named VM2 and has an IP address space of 10.10.0.0/24. You need to connect VNet1 to VNet2. What should you do first? 

A. Modify the IP address space of VNet2.
B. Move VM1 to Subscription2.
C. Provision virtual network gateways.
D. Move VNet1 to Subscription2

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Explanation:

When you need to connect two Vnets in different Subscriptions  here comes the concept of Virtual Network Gateway between Vnets. This is similar to creating Site to Site IP Sec connection to an on prem location. Both connectivity types use VPN Gateway to provide secure tunnel.
You can also connect Vnets via Vnet peering but it does not have VPN Gateway. Azure has two types of peering; Virtual Network Peering (Same Region) and Global Virtual Network Peering (Across Azure Regions).
For details please refer the link below.
Courtesy:  https://docs.microsoft.com/en-us/azure/vpn-gateway/vpn-gateway-howto-vnet-vnet-resource-manager-portal