A tuning fork vibrates with frequency 256Hz and gives one beat per second with the third normal mode of vibration of an open pipe. What is the length of the pipe?

The correct option is D 200cm
Frequency of tuning fork = 256 Hz
Frequency of open pipe = 256 ± 1 Hz
Frequency of third normal mode of vibration of an open pipe is ν = 3v/2l
where l = length of pipe
3 × 340 / 2 × l = 255
l = 200cm