Correct option (C) 20 cm
Explanation :
According to the questions f1 first overtone of a closed organ pipe
P1 = 3v/4I = 3 x V/4 x 30
f2 fundamental frequency of a open organ pipe P2 = V/2I2
Here given 3V/4 x 30 = V/2I2
I2 = 20 cm
Length of the pipe P2 will be 20 cm.