in this setup 3 hosts are trying to connect to single switch and since the switches forward packets based on MAC addresses, not ip addresses. However, for a communication to succeed, the hosts must be on the same subnet.

Why They Must Be in the Same Subnet

• When a host wants to send data to another device, it checks whether the destination IP address is in the same subnet by comparing it with its own IP address and subnet mask.
• If the destination is in the same subnet, the device sends the frame directly to the switch, which forwards it to the correct port.
• If the destination is in a different subnet, the device sends the frame to its configured gateway (a router).

In this case, since no router is present, all devices must be in the same subnet to communicate.

so the solution of this level is :

since the host C have the mask 255.255.255.128 they all should have the same mask and since the host A have the ip 104.198.14.125 as ip so the two other host should have the same 3 first bytes from that ip (the network portion of the ip) and any other host byte within the range.