Verified Answer :

For this reaction, oxidation number of Carbon in
CO would be +2 while oxidation number of carbon in CO2 would be +4 and so this
means that carbon has oxidized. Oxidation number of nitrogen in NO is +2. While
oxidation number of nitrogen in N2 is 0 so this means that nitrogen had reduced.
The reducing agent is the one which provides electrons by oxidizing itself so
in this case; CO is the reducing agent while the C in CO oxidized to produce
electrons. 
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