Step 1: Understanding the Concept
For a closed pipe the fundamental frequency is \(f=\dfrac{v}{4L}\), so \(f\propto\dfrac{1}{L}\).
Step 2: New length
Water fills \(\dfrac{1}{5}\) of the pipe, so the air column is \(L'=\dfrac{4L}{5}\).
Step 3: New frequency
\(f'=\dfrac{v}{4L'}=\dfrac{5}{4}f\).
Step 4: Percentage change
\(\dfrac{f'-f}{f}\times100=25\%\), option (A).
Final Answer:
The frequency increases by 25 percent.
\[ \boxed{\text{(A) }25\%} \]