reaction number 3 will not occur because magnesium nitrate is a salt and zinc is a metal and a metal and a salt cannot react
but the others two i am not sure
State "does not occur" if the reaction will not take place.
please let me know if i am right
a) Na + HNO3 ---> NaOH + NO2
b) Ni(II) + Cu2O --> Cu + NiO
c) Zn + Mg(NO3)2 --> does not occur