The ratio of ages of father to son is 13 : 2. Eighteen years hence, the ratio of their ages will be 2 :…

The correct choice is: 4 years

Let the ages be $13k$ and $2k$. After 18 years: $13k + 18 = 2(2k + 18)$.

$13k + 18 = 4k + 36 \Rightarrow 9k = 18 \Rightarrow k = 2$.

Son's age $= 2k = 4$ (father 26; after 18 years 44 and 22, ratio 2 : 1). Hence the answer is 4 years.