The correct option is B hydrogen
We know that pitch depend on the frequency of sound.
For open organ pipe, fundamental frequency,
f=v2l, ...........(1)
where l is the length of an organ pipe.
Also, speed of sound in gaseous medium,
v=√γRTM ...........(2)
At constant temperature and for diatomic gases,
f∝1√M
[from (1) and (2)]
So, lighter gas will have higher frequency thus higher pitch.