c
(c) In stretching $R \propto {l^2}$ $ \Rightarrow $ $\frac{{{R_2}}}{{{R_1}}} = \frac{{{l_2}^2}}{{{l_1}^2}}$ $ \Rightarrow $ $\frac{{{R_2}}}{{{R_1}}} = {\left( {\frac{2}{1}} \right)^2}$
$ \Rightarrow $${R_2} = 4{R_1}$. Change in resistance $ = {R_2} - {R_1} = 3{R_1}$
Now, $\frac{{{\rm{Change}}\,\,{\rm{in}}\,\,{\rm{resistance}}}}{{{\rm{Original}}\,\,{\rm{resistance}}}} = \frac{{3{R_1}}}{{{R_1}}} = \frac{3}{1}$