A leftward shift of a product supply curve might be caused by d) some firms leaving an industry.
When firms exit an industry, the overall supply of the product decreases. This reduction in the number of firms in the market leads to a leftward shift of the supply curve, indicating that at any given price, less quantity of the product is available. In contrast, options (a), (b), and (c) tend to increase supply or do not directly affect it; an improvement in production techniques and a decline in input prices typically lead to a rightward shift, while an increase in consumer incomes usually affects demand rather than supply.