Explanation:

- The current ages of Mr. Kumar and his son are in the ratio of 5:3.

- Let their present ages be 5x and 3x respectively.

- Fifteen years ago, their ages were (5x - 15) and (3x - 15).

- At that time, their age ratio was 2:1.

- So, (5x - 15)/(3x - 15) = 2/1.

- By solving this equation: 5x - 15 = 2(3x - 15), you get 5x - 15 = 6x - 30.

- Simplified, x = 15.

- Thus, Mr. Kumar's current age is 5x = 75 years.

- His son's current age is 3x = 45 years.

- Mr. Kumar was 75 - 45 = 30 years old when his son was born.

- Option:1 - 30 is the correct answer.