Paramagnetic Substances
Paramagnetic substances are those substances that have unpaired electrons. These unpaired electrons are attracted by an external magnetic field and cause the substance to be weakly attracted to the magnet.

Explanation of Options
Let's analyze each option to determine which one is paramagnetic.

a) NO (Nitric Oxide)
Nitric oxide (NO) has an odd number of electrons, which means it has one unpaired electron. Therefore, NO is paramagnetic.

b) O3 (Ozone)
Ozone (O3) has a total of 18 electrons. When we draw the Lewis structure for O3, we find that it has a double bond between one of the oxygen atoms, resulting in a resonance structure. Each oxygen atom has 6 electrons, and with the double bond, one oxygen atom has 7 electrons. Since all the electrons are paired, O3 is diamagnetic, not paramagnetic.

c) CO (Carbon Monoxide)
Carbon monoxide (CO) has a triple bond between the carbon and oxygen atoms. Carbon has 4 valence electrons, and oxygen has 6 valence electrons. The triple bond between them accounts for a total of 10 electrons. All the electrons are paired, making CO diamagnetic, not paramagnetic.

d) N2 (Nitrogen)
Nitrogen (N2) has a triple bond between two nitrogen atoms. Each nitrogen atom has 5 valence electrons. The triple bond accounts for a total of 6 electrons. Since all the electrons are paired, N2 is diamagnetic, not paramagnetic.

Conclusion
Among the given options, only NO (Nitric Oxide) has an unpaired electron, making it paramagnetic. The other options, O3, CO, and N2, have all their electrons paired, so they are diamagnetic.