The correct option is C 6
In an organ pipe only odd harmonics are found. So, frequency equal.to odd multiple of 1500Hz may be found in this case. Maximum audible frequency is 20,000Hz. So, possible frequency are 1500Hz, 4500Hz, 7500Hz, 10500Hz, 13500Hz, 16500Hz, 19500Hz. 19500Hz will be sixth overtone as 4500 will be first overtone.