An exogenous decrease in the velocity of money causes the aggregate demand curve to shift downward. In the short run, prices are fixed, so output falls.
If the Fed wants to keep output and employment at their natural-rate levels, it must increase aggregate demand to offset the decrease in velocity. By increasing the money supply, the Fed can shift the aggregate demand curve upward, restoring the economy to its original equilibrium point. Both the price level and output would remain constant.
If the Fed wants to keep prices stable, then it wants to avoid the long-run adjustment to a lower price level. Therefore, it should do precisely what Fed B does, and increase the money supply to shift the aggregate demand curve upward, again restoring the original equilibrium point.