Two natural numbers whose sum is 72 cannot be in the ratio:

A. 1:2
B. 3:5
C. 1:3
D. 3:4

72 should be divisible by addition of the above ratio.
Using trial and error method
For (a): – 1+2 = 3, 72 is divisible by 3
For (b): – 3+5 = 8, 72 is divisible by 8
For (a): – 1+3 = 4, 72 is divisible by 4
For (a): – 3+4 = 7, 72 is not divisible by 7
Therefore, the answer is option (d)