Your question is Troubleshooting a Complex Topology. Take a moment with it on the right.
Talk me through your thinking if you like. When you're confident, submit your answer and I'll grade it like a real screen (7/10 or better passes).
Describe how you would troubleshoot a network topology involving subnet masks, VLANs, STP, HSRP, EtherChannel, OSPF virtual links, and packet flow.