Final answer:
The best representation of the allylic cation shows the delocalization of the positive charge over adjacent carbon atoms. It's a hybrid of contributing resonance structures often depicted with dotted lines or a circle in chemical diagrams.
Step-by-step explanation:
The best representation of the allylic cation is that of a carbocation in which the positive charge is delocalized over two adjacent carbon atoms. In a simple allylic system such as propene, when one of the hydrogen atoms attached to the allylic carbon (the carbon adjacent to the double bond) is removed, the excess positive charge can resonate between the two terminal carbons of the allylic system. This delocalization of the positive charge provides some stabilization to the carbocation, making it more stable than a typical secondary carbocation.
In the resonance structures, the positive charge shifts from one carbon to another and the pi electrons of the double bond move to accommodate the shift in charge—resulting in different contributing structures. The actual structure of the allylic cation is a hybrid of these contributing resonance structures. In chemical diagrams, this is often represented by dotted lines or a circle within the allylic system to indicate the delocalized electrons.