When NaNO3  is heated in a closed vessel, oxygen is liberated and NaNO2  is left behind. At equilibrium

2  NaNO3(s)     ⇌     2 NaNO2(s)   +   O2(g) Appling law of Chemical Equilibrium. K=[O2(g)].   Hence addition of NaNO2  or NaNO3  will not disturb the equilibrium i.e.  (a)  and (b) are wrong. Only (c) and (d) are correct.