The property that justifies Step 1 in solving the equation \( 6x + 2(x - 1) = 30 \) is the Distributive Property.
In Step 1, the expression \( 2(x - 1) \) is being simplified to \( 2x - 2 \) by distributing the 2 across the terms inside the parentheses.
So, the correct response is: A. Distributive Property