Final answer:
An increase in the cost of producing the good could explain the shift in supply from S1 to S2. Therefore, the correct option is A.
Step-by-step explanation:
An increase in the cost of producing the good, option a), could explain the shift in supply from S1 to S2. When the cost of production increases, it becomes more expensive for sellers to produce the good. As a result, they may reduce the quantity they are willing and able to supply at each price level, leading to a leftward shift in the supply curve.