A sigma bond is formed by the head-on (axial) overlap of atomic orbitals. The internuclear axis is defined as the axis along which the nuclei of the two atoms lie. For a sigma bond to form, the orbitals must overlap directly along this internuclear axis.

Given that the z-axis is the internuclear axis:

• p_z – p_z overlap: When two p_z orbitals overlap head-on along the z-axis, they form a sigma bond. This is a direct, axial overlap.
• p_x – p_x overlap: If the z-axis is the internuclear axis, p_x orbitals are perpendicular to it. Their side-on overlap forms a pi (π) bond, not a sigma bond.
• p_y – p_y overlap: Similarly, p_y orbitals are also perpendicular to the z-axis. Their side-on overlap forms a pi (π) bond, not a sigma bond.
• p_x – p_z overlap: This type of overlap is not possible for bond formation as the orbitals are oriented differently and cannot achieve effective overlap along the internuclear axis. It would lead to zero net overlap.

Therefore, only the head-on overlap of p_z orbitals along the z-axis results in a sigma bond.

Correct Answer: (C)