Paramagnetism
Paramagnetism is a type of magnetism that occurs in substances that contain unpaired electrons. These unpaired electrons are attracted to an external magnetic field and cause the substance to be weakly magnetic. In contrast, diamagnetic substances have all their electrons paired and are repelled by a magnetic field.

Carbocation
A carbocation is a positively charged carbon atom, which has only three bonds and an empty p orbital. Since it has an empty orbital, it can accept an electron and become paired, making it diamagnetic. Therefore, carbocations are not paramagnetic.

Free Radical
A free radical is a molecule or atom that contains an unpaired electron. This unpaired electron makes free radicals highly reactive and unstable. Since free radicals have unpaired electrons, they can be attracted to an external magnetic field and are therefore paramagnetic.

Carbene
A carbene is a molecule that contains a divalent carbon atom, which means it has two unpaired electrons. These unpaired electrons make carbenes paramagnetic, as they can be attracted to an external magnetic field.

Nitrene
A nitrene is a molecule or a reactive intermediate that contains a divalent nitrogen atom with one unpaired electron. The presence of the unpaired electron in nitrenes makes them paramagnetic.

Conclusion
Based on the explanations above, the species that are not paramagnetic are carbocations (option A), carbenes (option C), and nitrenes (option D). Carbocations have an empty p orbital and can become paired, while carbenes and nitrenes have unpaired electrons, making them paramagnetic. Free radicals, on the other hand, contain unpaired electrons and are paramagnetic.