Answer: D.
Explanation:
For the supply to shift left, there must be a major development in the supply chain that will affect supply enough to decrease it for every price supplied.
Looking at the answers:
A. an increase in technology: this would not cause supply to be more expensive.
B. a decrease in the cost of a substitute would decrease demand, not supply.
C. an increase in consumer income levels would increase demand, as more consumers would be demanding more of the produce or service.
D. an increase in the cost of inputs for widgets would DECREASE supply, as the production of widgets is made more expensive at the same cost.
D is the correct answer.
