Final answer:
The correct justification for a proof step where one equal quantity is replaced by another is the Substitution Property, a fundamental concept in algebraic proofs.
Step-by-step explanation:
In mathematics, when you see an equation or expression where a quantity or term is replaced by another equal quantity or term, it is often due to the Substitution Property. This property allows for one quantity to be substituted for another equal quantity within an algebraic expression or equation. In the context of the proof you're referring to, if the eighth step involves replacing one expression with another equivalent expression, the correct justification would likely be the Substitution Property.
For example, if we have established that x = y, and we have another expression where x appears, we can substitute y for x. This does not affect the truth or the equality of the expression. It's a fundamental tool used to simplify and solve equations.