b
(b) \(l = \frac{{FL}}{{AY}} \Rightarrow l \propto \frac{L}{{{r^2}}}\) (\(F\) and \(Y\) are same)

\(\frac{{{l_2}}}{{{l_1}}} = \frac{{{L_2}}}{{{L_1}}}{\left( {\frac{{{r_1}}}{{{r_2}}}} \right)^2} = 2 \times {\left( {\frac{1}{2}} \right)^2} = \frac{1}{2}\)==>\({l_2} = \frac{{{l_1}}}{2} = \frac{l}{2} = 0.5mm.\)