The main axis of a diatomic molecule is designated as the z-axis. For effective overlap between atomic orbitals to form molecular orbitals, the orbitals must have the correct symmetry and orientation.

When the internuclear axis is the z-axis, p-orbitals can overlap in two ways:

1. Head-on (axial) overlap: $p_z$ orbitals on two atoms overlap along the internuclear axis to form $\sigma$ molecular orbitals.
2. Sideways (lateral) overlap: $p_x$ orbitals on two atoms overlap sideways to form $\pi$ molecular orbitals, and similarly, $p_y$ orbitals on two atoms overlap sideways to form another set of $\pi$ molecular orbitals.

However, $p_x$ and $p_y$ orbitals are perpendicular to each other and also perpendicular to the $p_z$ orbital (which lies along the internuclear axis). Therefore, a $p_x$ orbital from one atom cannot effectively overlap with a $p_y$ orbital from another atom when the internuclear axis is the z-axis. Their symmetries do not match for effective bonding overlap.

Option Analysis:

• A) $\pi$ molecular orbital: $\pi$ molecular orbitals are formed by the sideways overlap of parallel p-orbitals (e.g., $p_x$ with $p_x$, or $p_y$ with $p_y$). Overlap between $p_x$ and $p_y$ is not possible.
• B) $\sigma$ molecular orbital: $\sigma$ molecular orbitals are formed by head-on overlap (e.g., $s$ with $s$, $s$ with $p_z$, or $p_z$ with $p_z$). Overlap between $p_x$ and $p_y$ cannot form a $\sigma$ bond.
• C) $\delta$ molecular orbital: $\delta$ molecular orbitals are formed by the quadruple overlap of d-orbitals (e.g., $d_{xy}$ with $d_{xy}$). This is not relevant for p-orbital overlap.
• D) No bond will form: Due to the orthogonal nature and different symmetries of $p_x$ and $p_y$ orbitals with respect to each other and the internuclear axis, there will be no net constructive or destructive overlap, hence no bond will form.

Correct Answer: (D)