Once you call Activity B from Activity A, the system will execute Actitivity B. When Activity C is branched from Activity B, the system will execute Activity C. Once the Activity C is executed, the system will not procees to execute the rest of the steps in Activity B after the branch statement, instead it will go back to the point where Activity B is called. viz, Activity A. So the system will start executing the steps after the Call ActivityB step in ActivityA.
In the screen shot you can see that the step 3 of ActivityB is skipped since ActC is branched in Step2.