Dear Student
a.) Neutral gas ----> O2
Yellow non-metal -----> S
Balanced equation -
3S + 4O2 ----> SO2 + 2SO3
b.) Neutral gas ----> O2
Monovalent metal ----> Na (with valency equal to 1)
Balanced equation is -
6Na + 2O2 -----> 2Na2O + Na2O2
c.) CO2 and H2O reacts to form Carbonic Acid, H2CO3 (a weak acid)
Balanced equation -
CO2 + H2O ------> H2CO3
Regards