The correct option is A 200 Hz
∴ Frequency =vλ1=v2l ∴v1=v2l ...(i)
For closed pipe, vibrating in third harmonic, Frequency = 3v4l
or v2=3v4l
v2−v1=100 (given)
∴3v4l−v2l=100 or v4l=100 or v=400 l
∴v1=v2l=400l2l=200Hz
∴ Fundamental frequency of open pipe = 200 Hz