The correct answer is option 3: both carbon dioxide and bicarbonate salts.

Acidity regulators are substances added to drinks to control and balance their acidity levels. Carbon dioxide and bicarbonate salts are both commonly used as acidity regulators in soft drinks.

Carbon dioxide is a gas that is dissolved into the drink, creating carbonic acid. This adds a slightly tangy or acidic taste to the beverage. Carbon dioxide also provides the characteristic fizz or bubbles in carbonated soft drinks.

Bicarbonate salts, such as sodium bicarbonate or baking soda, are alkaline substances that can neutralize acidity. They work by reacting with the acidic components of the drink and reducing its overall acidity. This helps to create a more balanced and pleasant taste.

In conclusion, soft drinks often use both carbon dioxide and bicarbonate salts as acidity regulators. Carbon dioxide provides the tangy taste and bubbles, while bicarbonate salts help to balance and control the acidity levels of the drink.