The correct 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 answer is:
Addition Property of Equality.