Answer:
Given,
A's position from left = 13
A's position from right = 11.
Total number of boys in the row = Position of A from left + position of A from right - 1
= 13 + 11 - 1
= 23
So, there are 23 boys in the row.
Given, position of D from right = 17.
Let position of D from left be x
Total number of boys in the row = Position of D from left + position of D from right - 1
23 = x + 17 - 1
x = 23 - 17 + 1
= 7.
So, D is seventh from the left.
Hence, the correct answer is "7th".