1. The problem is to simplify the expression $$(\sqrt{12} + \sqrt{z})(\sqrt{12} - \sqrt{z})$$ assuming all variables are positive. 2. This expression is a product of conjugates, which follows the formula $$(a+b)(a-b) = a^2 - b^2$$. 3. Here, $a = \sqrt{12}$ and $b = \sqrt{z}$. 4. Applying the formula: $$ (\sqrt{12})^2 - (\sqrt{z})^2 $$ 5. Simplify each term: $$ 12 - z $$ 6. Therefore, the simplified expression is $12 - z$. The correct answer is option D.