The correct options are
A The reaction is endothermic.
C If NO2(g) and N2O4(g) are mixed at 400 K at partial pressures 20 bar and 2 bar respectively, more N2O4(g) will be formed.
D The entropy of the system increases.
As the temperature increases from 400 K to 500 K, the value of K increases from 50 to 1700.
Thus with increase in temperature, the equilibrium shifts to right.
This is possible for an endothermic reaction because, the heat absorbed will nullify the effect of increased temperature.
Hence, the forward reaction is endothermic.
The expression for the reaction quotient is Q=[NO2]2[N2O4].
Substitute values in the above expression.
Q=[20]2[2]=200
It is greater than the value of equilibrium constant which is 50. Thus more of reactant (N2O4) will be formed.
During the reaction, the number of moles increases from reactants to products. Hence, the entropy of the system increases.