Why is resonance is intramolecular process?

intramolecular - the process happens withing a same molecule ...

here in resonance delocalisation of pi electrons take place within a molecule so this is

a intramolecular process , same as intramolecular hydrogen bonding which occures within

a molecule...

Dear student,

In Benzene the two cyclohexatriene Kekulé structures first proposed by Kekule are taken together as contributing structures to represent the total structure. In the hybrid structure on the right the dashed hexagon replaces three double bonds, and represents six electrons in a set of three molecular orbitals of pie symmetry, with a nodal plane in the plane of the molecule.

The allyl cation  has two contributing structures with a positive charge on the terminal carbon atoms. In the hybrid structure their charge is +1/2. The full positive charge can also be depicted as delocalized among three carbon atoms.