Explanation:
Normally, you just need to create a /24 Virtual Network with minimum of 2 different zones /25 subnets.
/25 Subnet = (128 - 5) IPs : 123 available IPs
=> 50 VMs in zone A, 50 VMs in zone B
/25 Subnet = (128 - 5) IPs: 123 available IPs
=> 50 VMs in zone C
But,
the questions mentioned that 2 subscriptions. (Highlighted the "each subscription").
So, the minimum requirements should be:
1 of /24 Vnets for each subscriptions => 2 Vnets
2 of /25 subnets for each subscriptions => 4 subnets