Answer: a change in the price level.
Step-by-step explanation:
A shift in the aggregate supply curve is caused by non-price changes such as real wages of the workers, tax, technological innovation, productivity level etc.
The change in price will only result in the movement along the supply curve, which is also referred to as the change in quantity supplied. A change in price will not cause a shift on the aggregate supply curve.
Therefore, option A is the correct answer.