Concept: Covalent bonds are formed by the overlapping of atomic orbitals. These overlaps can be categorized into sigma ($\sigma$) bonds and pi ($\pi$) bonds, differing in their mode of overlap and resulting electron density distribution.

Why (D) is correct: A $\pi$ bond is formed by the sideways or collateral overlapping of unhybridized p-orbitals. This type of overlap results in electron density concentrated above and below the internuclear axis.

Option Analysis:

• A) By overlapping of hybridised orbitals: Hybridized orbitals primarily form $\sigma$ bonds through head-on overlap. While they can be involved in the framework of a molecule, $\pi$ bonds specifically require unhybridized p-orbitals.
• B) Overlapping of s-s orbitals: The overlapping of two s-orbitals always results in a $\sigma$ bond due to their spherical symmetry and head-on overlap.
• C) Head on overlapping of p-p orbitals: Head-on (axial) overlapping of p-p orbitals also forms a $\sigma$ bond, not a $\pi$ bond.
• D) By p-p collateral overlapping: This describes the sideways or parallel overlap of two unhybridized p-orbitals, which is the correct way a $\pi$ bond is formed.

Correct Answer: (D)