$\mathrm{R}=\frac{\mathrm{\rho \ell }}{\mathrm{A}}$         (∵ V = Aℓ conct.)

V = Aℓ

By differentiation                       0 = ℓdA + Adℓ                      ….(1)

By differentiation                 $\mathrm{dR}=\frac{\mathrm{\rho }(\mathrm{Ad\ell }-\mathrm{\ell dA})}{{\mathrm{A}}^2}$           ….(2) 

$\mathrm{dR}=\frac{2\mathrm{\rho d\ell }}{\mathrm{A}} \mathrm{or} \frac{\mathrm{dR}}{\mathrm{R}}=2\frac{\mathrm{d\ell }}{\mathrm{\ell }}$

So, $\frac{\mathrm{dR}}{\mathrm{R}}\%=2\frac{\mathrm{d\ell }}{\mathrm{\ell }}\%=2\times 0.1\%$

$\frac{\mathrm{dR}}{\mathrm{R}}\%=0.2\%$