Which of the following ingredients is used in the 18oz rocks watermelon margarita?

The 18oz rocks watermelon margarita prominently features grenadine, which is a non-alcoholic syrup made from pomegranate juice, and is commonly used in cocktails to add sweetness and color. In this specific margarita, grenadine enhances the overall flavor profile, contributing to the drink's refreshing and fruity character.

The other ingredients listed, such as mango puree, peach schnapps, and triple sec, are not typically associated with the watermelon margarita. While all of these ingredients can be found in various cocktails, they do not play a role in the signature mix of the rocks watermelon margarita, making grenadine the correct choice for this specific drink.