Acidified potassium dichromate (K$_2$Cr$_2$O$_7$) and alkaline potassium permanganate (KMnO$_4$) are potent oxidising agents employed in redox reactions to oxidise various substances. Oxygen, a strong oxidising agent, readily accepts electrons, facilitating combustion and oxidation. Conversely, hydrogen acts as a reducing agent by donating electrons. Thus, options (i), (ii), and (iii) are accurate.