$K_p/K_c$ for $N_2+3H_2\rightleftharpoons 2N{H}_3$ (gaseous phase) at 400 K is

$K_p=K_c(RT{)}^{\Delta n}$

$\Delta n=n_p-n_r$ = no. of moles of product & nr = no. of moles of reactant

 For reaction: $N_2+3H_2\rightleftharpoons 2N{H}_3$

$\Delta n=2-(1+3)=2-4=-2$

$K_p=K_c(R400{)}^{-2}=K_p/K_c=(400R{)}^{-2}$

Hence, the correct option is C i.e., $(400R{)}^{-2}$.