Goods market sings sweet harmony: supply's tune and demand's drum beat as one. Only "a)" holds the key: production's pulse matched by spending's hum.
Out of the options, only a) output is equal to aggregate demand is an essential feature of a goods market equilibrium. Here's why:
a) Output is equal to aggregate demand: This is the defining characteristic of equilibrium in the goods market. In this state, the total production of goods and services (output) exactly matches the total desired spending in the economy (aggregate demand). This balance ensures all available goods and services are being sold without excess supply or demand.
b) Investment is stable: While stable investment can be a consequence of equilibrium, it's not an essential feature. Equilibrium can occur with varying levels of investment as long as aggregate demand matches output.
c) The economy is at full employment: Similar to stable investment, full employment can be a result of equilibrium but isn't a strict requirement. Equilibrium can hold even with some unemployment if aggregate demand still aligns with output.
d) Output is fixed: This contradicts the very concept of equilibrium. Output can adjust in response to changes in demand or supply, reaching a new equilibrium point. A fixed output wouldn't allow for this adjustment.
Therefore, the only essential feature of a goods market equilibrium is a) output is equal to aggregate demand. The other options might be consequences or correlations of equilibrium, but they don't define it directly.
Remember, equilibrium can be a dynamic state subject to changes in external factors. These changes can push the market away from equilibrium, but the forces of supply and demand will typically work to restore the balance where output and aggregate demand are equal.