Substituting 1 for \( x \) in the equation \( 5x + 3 = x \cdot 5 + 3 \) is a test case for the Commutative Property of Multiplication.
In this equation, \( x \cdot 5 \) is equivalent to \( 5x \) (due to the commutative property of multiplication), which is demonstrated by testing with the substitution of \( x = 1 \).
So, the correct answer is:
The Commutative Property of Multiplication.