Answer:
The natural frequency of an organ pipe depends on the pipe length.
Step-by-step explanation:
An organ pipe is an instrument which is used to produce sound. The organ pipe are of two types i.e. closed pipe and organ pipe.
Mathematically, the frequency of closed organ pipe is given as :

For open organ pipe :

Where,
n is the number of overtones
v is the speed of sound
l is the length of the pipe.
Hence, the correct option is (a) " pipe length ".