A father is now three times as old as his son. Five years back he was four times as old as his son. The age of the son is

Option (a), 12 years, is incorrect because $12 \times 3 = 36$, which is not the father’s current age. Option (c), 18 years, is incorrect because $18 \times 3 = 54$, which is not the father’s current age. Option (d), 20 years, is incorrect because $20 \times 3 = 60$, which is not the father’s current age.