Correct Answer - Option 3 : 19
According to the question,
Pramod is 7th from left and Prakash is 9th from right.
After interchanging their positions,
Pramod is 11th from left.
Thus, total number of persons in a row = Pramod's position (New) + Prakash' position - 1
Thus, total number of persons in a row = 11 + 9 - 1 = 19
Hence, 19 is correct.