Step 1: Frequency relation for a stretched string.
For a sonometer wire, frequency is given by
\[
f \propto \frac{1}{L},
\]
when tension and linear density remain constant.
Step 2: Effect of increase in length.
If length is increased by 25%, then
\[
L' = 1.25L.
\]
Hence, new frequency becomes
\[
f' = \frac{f}{1.25} = 0.8f.
\]
Step 3: Change in second harmonic frequency.
Second harmonic frequency is directly proportional to the fundamental frequency. Therefore, it also becomes 80% of its original value.
Step 4: Conclusion.
The frequency decreases by 20%.