$\large\frac{k_2}{k_1}=\frac{R_1}{R_2}$
$\Rightarrow k_2=\large\frac{R_1}{R_2}$$\times k_1$
$\Rightarrow \large\frac{50}{280}$$\times 1.4Sm^{-1}$
$\lambda_m=\large\frac{k_2}{C}=\frac{1/4Sm^{-1}}{0.50mol L^{-1}}$
$\Rightarrow \large\frac{1}{4}\times \frac{10^{-3}}{0.50}$
$\Rightarrow 5\times 10^{-4}Sm^2mol^{-1}$
Hence (C) is the correct answer.