Which of the following events would cause a shift to the right in the supply curve?
a. the plant shuts down for several weeks because there's been a fire.
b. the manufacturer lowers the price of its candy bars.
c. a candy bar manufacturer raises the price of its candy bars.
d. upgrades to its mixing equipment allow the plant to make more bars.

The correct answer is D.

The upgrades in the physical capital used in the production process, have made possible to manufacture the product more efficiently (more bars obtained in the same plant) than before. Therefore, production costs are reduced, the producers would be willing to supply a larger amount than before at all price levels and the supply curve shifts to the right.