Carbocations are positively charged carbons. If a carbocation is attached to one carbon only, it is called primary. When a carbocation is attached to two carbons, it is secondary. When the carbocation is attached to three carbons, it is tertiary. The more substituted (connected to more carbons) carbocation is, the more stable it is. Therefore tertiary is more stable than secondary and more stable than primary. Furthermore, if a carbocation has resonance, it will be much more stable as well. The most common resonance is a carbocation next to a double bond. Choice C is a primary carbocation but with resonance (no other choices have resonance) and is the answer to the question below.
