
Explanation

Box 1: Network Contributor on RG1
To add to the backend pool, write permission is required on the Resource Group because it writes deployment information. To add a backend pool, you need network contributor role on the LB and on the VMs that will be part of the backend pool.
For this reason the network contributor role must be assigned to the RG where the LB and the VM resides. So the correct answer is Network Contributor on RG1 .
Box 2: Network Contributor on RG1
For Health Probe also, without having access to RG1, no health probe can be added. If only Network Contributor role is assigned to LB then the user would not be able to access the IP addresses of the member pools.
Owner/Contributor can give the user access for everything. So it will not fit into the the principle of least privilege. Hence Owner and contributor role is incorrect choices for the question.
Reference:
https://docs.microsoft.com/en-us/azure/role-based-access-control/built-in-roles