Which tool is specifically designed to add air to a batter?

The tool specifically designed to add air to a batter is the whisk. A whisk features multiple thin wires that create a lot of surface area, which effectively incorporates air into the mixture as it is stirred or beaten. This aeration is essential in many baking processes, as it helps to lighten the batter, enhancing the texture and contributing to the rise in baked goods.

The rubber spatula, while useful for folding in ingredients gently and scraping bowls, does not aerate mixtures in the same way. The slotted spoon is primarily used for lifting food items out of liquid, such as draining cooked pasta or vegetables, and does not serve an aerating purpose. The ladle is designed for serving liquids or semi-liquids but does not incorporate air into batters. Thus, the whisk stands out as the tool specifically suited for adding air into a batter, making it the correct choice.