When substituting regular milk for buttermilk, what additional ingredient should be added?

When substituting regular milk for buttermilk, the addition of an acid is essential to mimic buttermilk's characteristic tangy flavor and acidity. By adding lemon juice or vinegar to the milk, you create an acidic environment that reacts with the baking soda or baking powder in recipes, similar to how buttermilk does.

This reaction is crucial in baked goods as it helps to activate the leavening agents, resulting in a fluffy texture. Buttermilk not only adds acidity but also contributes to moisture and flavor, which can be effectively replicated by incorporating an acid like lemon juice or vinegar into the milk. This is why the correct answer involves adding an acid to the milk substitute, ensuring that the intended outcome of the recipe is maintained.