Explanation:

A+B/2 = 20 years
A + B = 40 years ......(i) 
C+B/2 = 19 years
C + B = 38 years ......(ii)
C+A/2 = 21 years
C + A = 42 years .....(iii)
Add equation (i), (ii) and (iii)
2 (A + B + C) = 120 years
A + B + C = 60 years .....(iv) 
From equation (i) and (iv)
40 + C = 60 
C = 20 
From equation (ii) and (iv) 
A + 38 = 60 
A = 22 years 
From equation (iii) and (iv) 
B + 42 = 60 
B= 18 years

Hence, option 4 is the correct answer.