Concept: Valence Bond Theory (VBT) describes chemical bonding as the overlap of atomic orbitals. The type and extent of orbital overlap determine the strength and nature of the chemical bond formed.

Why (A) is correct:

A) A $\sigma$-bond is formed by the head-on (axial) overlap of atomic orbitals. This type of overlap results in a greater extent of overlap compared to sidewise overlap. A greater extent of overlap leads to a stronger bond. A $\pi$-bond is formed by the sidewise (lateral) overlap of atomic orbitals, which is less effective than head-on overlap, making $\pi$-bonds generally weaker than $\sigma$-bonds. Therefore, $\sigma$-bond is stronger than $\pi$-bond.

Option Analysis:

• B) p-orbitals always have only sidewise overlapping. This is incorrect. p-orbitals can undergo both head-on (axial) overlap to form $\sigma$-bonds (e.g., in F${}_2$ where p-p overlap forms a $\sigma$-bond) and sidewise (lateral) overlap to form $\pi$-bonds (e.g., in O${}_2$ or N${}_2$).
• C) s-orbitals form $\pi$-bonds. This is incorrect. s-orbitals are spherical and can only undergo head-on overlap to form $\sigma$-bonds. They cannot form $\pi$-bonds because $\pi$-bonds require sidewise overlap of orbitals with parallel axes, which is not possible for s-orbitals.
• D) $\pi$-bond is stronger than $\sigma$-bond. This is incorrect. As explained in option A, $\sigma$-bonds are generally stronger than $\pi$-bonds due to more effective head-on overlap.

Correct Answer: (A)