The property of equality that justifies the step from \( 3z/2 - 5 = 10 \) to \( 3z/2 - 5 + 5 = 10 + 5 \) is the Addition Property of Equality. This property states that if you add the same value to both sides of an equation, the two sides remain equal.
So the correct response is:
Addition Property of Equality