b
$P \propto \frac{1}{R}$ and $ R \propto l \Rightarrow P \propto \frac{1}{l}$
$ \Rightarrow \frac{{{P_1}}}{{{P_2}}} = \frac{{{l_2}}}{{{l_1}}} \Rightarrow \frac{{{P_1}}}{{{P_2}}} = \frac{{(100 - 10)}}{{100}} = \frac{{90}}{{100}}$$ \Rightarrow {P_2} = 1.11\,{P_1}$
$\%$ change in power = $\frac{{{P_2} - {P_1}}}{{{P_1}}} \times 100 = 11\% $