The correct option is C The second harmonic of the pipe
Fundamental frequency (first harmonic frequency) of an open organ pipe is given by ,
n1=v/λ=v/2l ,
where v=speed of sound in air (=330m/s,given),
λ= wavelength ,
l= length of organ pipe (=33cm=0.33m , given) ,
therefore , n1=330/(2×0.33)=500Hz ,
as an open pipe produces even and odd harmonics , and given frequency is 1000Hz , it is 2 times of the first harmonic frequency , therefore it is second harmonic frequency of the pipe .