A. Fe + S → FeS
Here , Fe is oxidised from 0 to +2, while S is reduced from 0 to -2
Fe undergoes oxidation (as losing electrons)
Sulphur undergoes reduction (as accepting electrons)
B. 2Ag2O → 4Ag + O2
Ag in Ag2O is reduced from +1 to 0, and oxygen Ag2O is oxidised from -2 to 0
O undergoes oxidation (as losing electrons)
Ag undergoes reduction (as accepting electrons)
C. 2Mg + O2 → 2MgO
Mg is oxidised from 0 to +2, and O is reduced from 0 to -2
Mg undergoes oxidation (as losing electrons)
O2 undergoes reduction (as accepting electrons)
D. NiO + H2 → Ni + H2O
Ni is reduced from +2 tp 0, and H is oxidised from 0 to +1
H undergoes oxidation (as losing electrons)
Ni undergoes reduction (as accepting electrons)