The step in the equation:
\[ 3z/2 - 5 + 5 = 10 + 5 \]
is justified by 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. Thus, in this case, adding 5 to both sides helps isolate the term on the left side.
So, the correct answer is:
Addition Property of Equality.