c
(c) $\frac{Q}{{At}} = K\frac{{\Delta \theta }}{l}$ ==> $K\frac{{\Delta \theta }}{l}$= constant ==> $\frac{{\Delta \theta }}{l} \propto \frac{1}{K}$
Hence If ${K_c} > {K_m} > {K_g}$, then
${\left( {\frac{{\Delta \theta }}{l}} \right)_c} < {\left( {\frac{{\Delta \theta }}{l}} \right)_m} < {\left( {\frac{{\Delta \theta }}{l}} \right)_g} \Rightarrow {X_c} < {X_m} < {X_g}$
because higher $ K$ implies lower value of the temperature gradient.