Assertion (A): $\sigma$  electrons are referred as localized electrons.

Reason (R) :  $\pi$ - electrons are referred as mobile electrons.

In a chemical bond the sigma bond is formed by the overlapping of atomic orbitals through inter nuclear axis.  Hence sigma electrons are bound to in a specific direction thus those are called localized electrons.  But the pi-bond is formed by side wise overlapping of atomic orbitals above and below the inter nuclear axis of two atoms.  Hence these pi electrons are move easily round the atom.  Hence these are called mobile electrons or de localized electrons.