Final answer:
A free radical is paramagnetic because it has an unpaired electron and is attracted to a magnet. Carbonium ions and carbanions do not typically have unpaired electrons, so they are not paramagnetic.
Step-by-step explanation:
The question at hand is which of the following species is paramagnetic: a carbonium ion, a free radical, a carbanion, or all of these. A species is considered paramagnetic if it contains at least one unpaired electron and is attracted to a magnet. A carbonium ion (carbocation) has a positive charge and lacks an unpaired electron; therefore, it is typically diamagnetic, not paramagnetic. A free radical has an unpaired electron, which makes it paramagnetic. A carbanion has a negative charge and typically an even number of electrons paired up, so it is not usually paramagnetic. Thus, the correct answer is b. A free radical.