Which of the following are reducing reagents? O2

In a reaction with CH4 to give CO2 and H2O it is oxidising and for most reactions known it is an oxidising agent.

Hypothetically it could be a reducing agent if it was to react with F2, producing 2 OF (which would be an unstable molecule).

C

(in a reaction with oxygen to give carbon monoxide)

Zn

(In a reaction with HCl to give ZnCl2 and hydrogen as the product)

H2

(in a reaction with oxygen to give water as the product)

KMnO4 (potassium permanganate)

(in a reaction with oxalate(C2O42-) to produce CO2 )

K2Cr2O7 (potassium dichromate)

in the reaction with ethanol (C2H6O) to give ethanal (C2H4O)