Which of the following gases is not a greenhouse gas?

1.  CO

2.  O3 

3.  CH4

4.  Water vapour

The gases which absorb sunlight near the earth's surface and then its radiated back to the earth are called green house gases.
Carbon dioxide, water vapour, methane, ozone, oxides of nitrogen, chlorofluoro carbons etc; are green house gases. CO is not a green house gas.