Explanation:

Since, the children were born at the interval of three years ,

let us assume their age as x, x+3,x+6,x+9,x+12.
given- the sum of five children is 50
therefore, 
x+x+3+x+6+x+9+x+12 = 50
5x+30 = 50
5x = 50-30
5x=20
x=4
Hence, age of the youngest child is 4 years.

Hence, option 1 is the correct answer.