What ingredient gives the Lemon Drop its tart flavor?

The ingredient that gives the Lemon Drop its tart flavor is lemon juice. This citrus component is essential because it provides the characteristic sourness that balances the sweetness of other ingredients, such as simple syrup or sweet and sour mix. The vibrant acidity of lemon juice is crucial in achieving the cocktail's bright and zesty profile, making it refreshing and lively.

In cocktails like the Lemon Drop, the balance between sweetness and tartness is key to creating a well-rounded drink. The use of lemon juice not only imparts flavor but also contributes to the drink's overall mouthfeel and complexity, enhancing the drinking experience. Without lemon juice, the drink would lack its signature tartness, making it less enjoyable and deviating from its classic formulation. Other options do not provide the same acidity or tartness that defines the Lemon Drop.