Can someone explain part (b)?

[dash_riprock]

It's the rule. When a pitch strikes a runner (it will always be a runner from 3rd stealing on the pitch), the ball is dead and all runners move up one base. It is still a pitch, and it is what it is. In this case it is strike three, so the batter is out on strikes (and can never advance). If it's the 3rd out, no run can score. Otherwise, count the run.