Which of these describes an item for which a change in its supply is proportional to a change in its price?

Answer: Unit Elastic

A unit elastic demand describes an item that assumes a change in its supply will cause an equal proportional change in its price or quantity demanded. For example; if a person raises the price of her product like apple pie by $1.00, the unit elastic demand for that one dollar increase would mean a decrease in the quantity demanded by one unit. Therefore, unit elastic demand defines an item that describes a demand or supply that is perfectly responsive to price changes by the same percentage.