Final answer:
When the government imposes a price floor above the equilibrium price, it leads to excess supply, a decrease in the quantity demanded, and an increase in the quantity supplied, hence option D is correct.
Step-by-step explanation:
If the equilibrium price of a jar of spaghetti sauce is $3 and the government imposes a price floor of $4 per jar, this introduces a situation where the price is set above the equilibrium price, which is the price where demand and supply meet. Price floors are designed to prevent a price from falling below a certain level, but when set above the equilibrium price, they lead to excess supply or surpluses because the quantity supplied will exceed the quantity demanded. As a result, the quantity demanded of spaghetti sauce decreases, and the quantity supplied increases, which is consistent with option D.