A purse contains 2 six-sided dice. One is a normal fair die, while the other has two 1s two 3s,and two 5s. A die is picked up and rolled. Because of some secret magnetic attraction of the unfair die, there is 75% chance of picking the unfair die and a 25% chance of picking a fair die. The dice is rolled and shows up the face 3. The probability that a fair die was picked up is (a) `1/7` (b) `1/4` (c) `1/6` (d) `1/24`