Assertion (A): In ${\text{H}}_2$  molecule, one electron pair is shared between two 'H' atoms.

Reason (R): Hydrogen atom requires one electron to acquire the stable electronic configuration of Helium.

Between two hydrogen atoms covalent bond is present.  This is formed by mutual sharing of electrons between two hydrogen atoms. Each  hydrogen contributes one electron and form a pair of electrons between two hydrogen atoms.  This pair equally shared by both the hydrogen atoms and gets helium configurations of two hydrogen atoms.