The equation \( 5x + 3 = x \cdot 5 + 3 \) demonstrates the Commutative Property of Multiplication because it shows that the order of multiplication does not affect the result (i.e., \( 5x \) is the same as \( x \cdot 5 \)).
Therefore, substituting 1 for \( x \) tests the Commutative Property of Multiplication.
The correct response is: The Commutative Property of Multiplication.