Final answer:
Grenadine is the ingredient responsible for the distinctive red color of the Singapore Sling, a popular cocktail.
Step-by-step explanation:
The ingredient that gives the Singapore Sling its distinctive red color is A) Grenadine. This classic cocktail combines gin, grenadine, cherry liqueur, fresh lemon juice, and a blend of other liquors and flavors. Grenadine, a sweet red syrup made from pomegranates, is primarily responsible for the vibrant hue of the drink.