What is NOT a best practice for Mobile Application Architecture: Limit server side calls?

What is NOT a best practice for Mobile Application Architecture: Limit server side calls?
A . Limit Server side calls to online transactions or Synchronization. Do the bulk of the operations on the Client Side before passing information to the Server Side.
B . Synchronize on every screen or online event for better granularity No direct calls to backend systems. Do call to Outsystems Core Services and Outsystems will handle the interaction with external backend systems.
C . Direct calls is bad from Security, Performance and Maintainability standpoint. Direct calls will require javascript Code and javascript Code is harder to maintain.

Answer: B