Karen is 50% faster than John
So by the time John covers 20% of the total distance, Karen would have covered 20% + (50% of 20%)
= 20% of 10% = 30% and 20 + 30 = 50%

Without John having cramps; for every 50% of the total distance, John will cover 20% and Karen will cover 30%

For the whole (50% * 2 = 100%) John will cover (20% * 2 = 40%) and Karen will cover (30% * 2 = 60%)
But after covering 25% of the total distance, John gets a cramp. So, Karen covered the remaining (100% - 25% = 75%) of the total distance

The percentage increase from what Karen should have covered if John didn't get cramps to what she now has to cover before she can meet with John = $$\left(\frac{new\ dis\tan ce-original\ dis\tan ce}{original\ dis\tan ce}\right)\ \cdot\frac{100}{1}$$
$$\left(\frac{75\%-60\%}{60\%}\right)\ \cdot\frac{100}{1}$$
$$=\frac{15\%}{60\%}\ \cdot\frac{100}{1}$$
$$=0.25\ \cdot\ \frac{100}{1}=25\%$$

Answer = A