The correct options are
B C+O2→CO2
C 2C+O2→2CO
D 2N2+O2→2N2O
An oxidizing agent (also called oxidant or oxidizer) is the element or compound in an oxidation-reduction (redox) reaction that accepts an electron from another species. Since the oxidizing agent gains electrons (and is thus, often called an electron acceptor), it is said to have been reduced.
Hence, options B, C & D are correct where oxygen is getting reduced.