Choice "D" is correct. When potential national income (potential Gross Domestic Product, also referred to as the Long Run Aggregate Supply) is greater than the achieved national income, the short run aggregate supply curve is shifting to the left indicating a contraction or recession.
Choice "a" is incorrect. Declining unemployment (increasing employment) is indicative of a growing economy, not a recession.
Choice "b" is incorrect. Declining purchasing power is usually a sign of inflation. Inflation and, usually, the related occurrence of increased money supply are typically indicative of economic growth and not recession.
Choice "c" is incorrect. Shortages of raw materials are generally indicative that the economy is growing, not shrinking, and is placing greater demand on limited raw material resources than available capacity.