Final answer:
A pi bond is created by the side-by-side overlap of two p orbitals, characterized by the electron density on opposite sides of the internuclear axis and a nodal plane where no electrons can be found. So the correct option is a.
Step-by-step explanation:
A pi bond (π bond) is a type of covalent bond that forms from the side-by-side overlap of two p orbitals. This side-by-side overlapping allows the electron density to be found on opposite sides of the internuclear axis, which is the line between the two nuclei where the bonding occurs. A pi bond is characterized by a nodal plane containing this internuclear axis, with no probability of finding an electron within that plane. This differentiates it from a sigma bond, which involves end-to-end overlap and has its electron density concentrated along the internuclear axis.