Consider the following substances :
(i) Acidified K₂Cr2O7
(ii) Alkaline KMnO4
(iii) Oxygen
(iv) Hydrogen

Out of these, the commonly used oxidising agents are:
(a) (i) and (ii) only
(b) (ii) and (iii) only
(c) (i), (ii) and (iii)
(d) (i), (ii) and (iv)​