CrO₃ in anhydrous medium can be used to oxidize primary alcohols to aldehydes. Here, CrO₃ acts as an oxidizing agent. Hence, option (i) is correct.

Pyridinium chlorochromate (PCC) can also be used to oxidize primary alcohols to their corresponding aldehydes. Hence, option (iii) is correct.

Primary alcohols can also be oxidized to their corresponding aldehydes by dehydrogenation or heating them in the presence of Cu at 573K. Hence, option (iv) is correct.

We know,KMnO₄ is a strong oxidizing agent. Thus, when Primary alcohols are treated with KMnO₄ in acidic medium,they are oxidized to a mixture of carboxylic acids. Hence, Option (ii) is incorrect.