Assertion (A) : $\text{s}-\text{s}$ Overlapping is present in ${\text{H}}_2$ molecule.

Reason (R) : ${\text{H}}_2$ Molecule is formed by the overlapping of’1s’ orbitals of 2 hydrogen atoms.

Valence bond theory explains overlapping of atomic orbitals.  Based on this theory ${\text{H}}_2$ molecule contains two hydrogen atoms.  Its electronic configuration is ${\text{1s}}^{\text{1}}$.  1s orbital’s of two hydrogen atoms partially merged to form ${\text{H}}_2$ molecule.