I generally understand the risk and mitigation.
However, from exam perspective I am not very clear on this.
Security, Network failure etc are the general risk to all the applications.
Epractize labs is also not clear on this e.g. it simply says that "application needs to be scalable".
If I think too far, even earthquack, tsunami could be the risk and make the entire data center go down.
In the exam you need to put the top 3 risks related to the proposed architecture. earthquack, tsunami are risks that can happen, but I believe they are not the most worrisome to be mentioned in your solution. Security (Session Hijack, MIM, brutal force...), communication failures, performance, scalability, availability and maintanabilitie are always on the list of most concern ....