(a) The electrical resistivity of iron is `10.0 xx 10^(-8) Omegam` whereas that of mercury is `94.0 xx 10^(-8) Omegam`. Since the resistivity of iron is less than that of mercury, therefore, iron is a better conductor than mercury.
(b) Silver metal has the lowest electrical resistivity (of `1.60 xx 10^(-8) Omega m)`, therefore, silver metal is the best conductor of electricity.