The equation \( 4(3+5) = 12+20 \) is an application of the Distributive Property. This property states that when you multiply a number by a sum, you can distribute the multiplication to each addend within the parentheses. In this case, \( 4(3+5) \) can be distributed as \( 4 \cdot 3 + 4 \cdot 5 \), resulting in \( 12 + 20 \).
So the correct response is: Distributive Property.