d
(d) \(\frac{Q}{t} = \frac{{KA\,\Delta \theta }}{l}\)

==> \(\frac{Q}{t} \propto \frac{A}{l} \propto \frac{{{d^2}}}{l}\) ( \(d =\) Diameter of rod)

==> \(\frac{{{{(Q/t)}_1}}}{{{{(Q/t)}_2}}} = {\left( {\frac{{{d_1}}}{{{d_2}}}} \right)^2} \times \frac{{{l_2}}}{{{l_1}}} = {\left( {\frac{1}{2}} \right)^2} \times \left( {\frac{1}{2}} \right) = \frac{1}{8}\)