Which ingredient is necessary when making bulk lemonade?

Lemon juice is the fundamental ingredient when making bulk lemonade. It provides the primary flavor and tartness that defines lemonade. Without lemon juice, the beverage would lack its essential character, as it is the key component that gives lemonade its distinct taste.

While the other options enhance or complement the drink (such as green tea providing a different flavor profile, soda water adding fizziness, and ice cubes serving to chill the beverage), none can replace the crucial role of lemon juice in creating an authentic lemonade. Hence, lemon juice is indispensable in the preparation of this refreshing beverage.