5,12,20,29,39,__
Here, the difference between 5 and 12 is 7.
And the difference between 12 and 20 20 is 8.
And the difference between 20 and 29 is 9.
And the difference between 29 and 39 is 10.
Since the difference is incredible by 1.
And the next number could be 50.
Because the difference between 39 and 50 is 11 which is the next number of previous difference.
Option d is correct.