Substituting 1 for \( x \) in the equation \( 5x + 3 = x \cdot 5 + 3 \) tests the Commutative Property of Multiplication.
In this case, the equation demonstrates how the multiplication of \( x \) by 5 can be reordered (i.e., \( 5x = x \cdot 5 \)). Therefore, the correct response is:
The Commutative Property of Multiplication.