A sufficiently long closed organ pipe has a small hole at its bottom. Initially the pipe is empty. Water is poured into the pipe at a constant rate. The fundamental frequency of the air column in the pipe

Fundamental frequency of a closed pipe is given by $f_0=\frac{v}{4\mathcal{l}}$
Here, v = speed of sound in air.
Length  $\mathcal{l}$ of air column will first decrease and then becomes constant (when rate of inflow = rate of outflow). Therefore, $f_0$will first increase and then become constant.