Hello

The correct option is (c ) dear
It is a simple formula based problem, which we can solve in this way:-


First of all, we know that fundamental Frequency for the Open orange pipe is

=1 /2l'

where, = Frequency and l' is the length of the open orange pipe.



Now, Third Harmonic of the orange pipe = 3/4l

where, = frequency

and l is the length of the Closed orange pipe = 20 cm.


Now, According to the Question,


3/4l = /2l'

3/(4 20) = 1/2l'
3/80 = 1/2l'
2l' = 80/3
l' = 40/3
l' = 13.33 cm.


Hence, the length of the open orange pipe is 13.33 cm.
I hope that it helps you.

Good luck for your examination

Thankyou