A certain wire A has resistance 81 $\mathrm{\Omega }$. The resistance of another wire B of same material and equal length but of diameter thrice the diameter of A will be:

Resitance of a wire is given by, $\mathrm{R}=\rho \frac{\ell }{\mathrm{A}}$
($\rho$ is specifi c resistance, $\mathcal{l}$ is length and A is area of cross–section πr²)
Here, material and length are same.
So, $R\propto \frac{1}{A}$
$\begin{array}{l}\frac{R_1}{R_2}=\frac{A_2}{A_1}=\frac{\pi r_2^2}{\pi r_1^2}=\frac{r_2^2}{r_1^2}={\left(\frac{3}{1}\right)}^2=9\\ R_2=\frac{R_1}{9}=\frac{81}{9}=9\mathrm{\Omega }.\end{array}$