Correct Answer - Option 4 : 3 years

**Given **

Sum of father's age and mother's age = 51 years

**Calculation **

Let the age of the son is z

Father’s age = 9z and mother’s age = 8z

So, 9z + 8z = 51 years

⇒ 17z = 51 years

⇒ z = 3 years

**∴ The age of the son is 3 years**