[
  "Let the cocktail be 24 units (LCM of 4 and 6). Grape juice = 1/4 = 6 units. Remaining = 18 units.",
  "Orange juice = 5/6 of 18 = 15 units. Pineapple juice = 18 - 15 = 3 units. Fraction = 3/24 = 1/8.",
  "Pineapple juice is 1/8 of the cocktail. Answer: 1/8 (option B)."
]