Use the unit method. Let the cocktail be 24 units (LCM of 4 and 6).<br><br>
&bull; Grape juice: \( \frac{1}{4} \) of total = 6 units<br>
&bull; Remaining: 24 − 6 = 18 units<br>
&bull; Orange juice: \( \frac{5}{6} \) of remaining = \( \frac{5}{6} \times 18 \) = 15 units<br>
&bull; Pineapple juice: 18 − 15 = 3 units<br>
&bull; Fraction of pineapple juice: \( \frac{3}{24} = \frac{1}{8} \)<br><br>
Final Answer: B: \( \frac{1}{8} \)