Substituting 1 for \( x \) in the equation \( 5x + 3 = x \cdot 5 + 3 \) is a test case for the Commutative Property of Multiplication.
This is because the equation shows that the order of multiplication does not matter: \( 5x \) is the same as \( x \cdot 5 \). Hence, the correct response is:
The Commutative Property of Multiplication.