The equation \( 5x + 3 = x \cdot 5 + 3 \) demonstrates the commutative property of multiplication.
In this equation, \( 5x \) and \( x \cdot 5 \) represent the same value, as the commutative property states that the order of factors in multiplication can be changed without affecting the product. Thus, the correct answer is:
D) commutative property of multiplication.