The equation \( 5x + 3 = x \cdot 5 + 3 \) demonstrates the Commutative Property of Multiplication. This property states that changing the order in which two numbers are multiplied does not change the product. Here, \( 5x \) can be rewritten as \( x \cdot 5 \), showing that multiplication is commutative. Therefore, substituting 1 for \( x \) tests this property.
So, the correct answer is:
The Commutative Property of Multiplication