b) An exogenous increase in the price of oil.
An exogenous increase in the price of oil is an adverse supply shock that causes the short-run aggregate supply curve to shift upward.
If the Fed cares about keeping output and employment at their natural-rate levels, then it should increase aggregate demand by increasing the money supply. This policy response shifts the aggregate demand curve rightward. In this case, the economy immediately reaches a new equilibrium – the price level is permanently higher, but there is no loss in output associated with the adverse supply shock.
If the Fed cares about keeping prices stable, then there is no policy response it can implement. In the short run, the price level stays at the higher level. The Fed must simply wait, holding aggregate demand constant. Eventually, prices fall to restore full employment at the old price level. But the cost of this process is a prolonged recession.