A polar covalent bond occurs between atoms that have a significant difference in electronegativity.

Electronegativity is an atom's ability to attract shared electrons towards itself. Due to this difference, the bonding electron pair (the electrons involved in the bond) spends more time closer to the more electronegative atom. This results in an unequal sharing of the electron pair, creating a partial negative charge (?−) on the more electronegative atom and a partial positive charge (?+) on the less electronegative atom.

B. equal sharing of electron pair: This describes a nonpolar covalent bond, which typically occurs between atoms of the same element (e.g., in O\(_2\) or H\(_2\)) where the electronegativity difference is zero or negligible.

C. equal sharing of electron lone pair: A lone pair (or unshared pair) is a pair of valence electrons that is not involved in bonding. The concept of "sharing" applies to bonding pairs.

D. unequal sharing of electron lone pair: Lone pairs are associated with a single atom and are not shared between atoms in the context of a bond, so this option is chemically inaccurate in describing the bond itself.

The correct option is A. unequal sharing of electron pair.