Which of the following combination leads to formation of π bond (Molecular axis is z axis)(A) py + py   (B) dyz + dyz(C) px + dzx (D) dxy + dyz

The formation of a π bond requires the sideways overlap of atomic orbitals. This means that the orbitals must be oriented in the same plane.

(A) py + py: This combination can form a π bond. The py orbitals are both in the y plane and can overlap sideways.

(B) dyz + dyz: This combination can also form a π bond. The dyz orbitals are both in the yz plane and can overlap sideways.

(C) px + dzx: This combination cannot form a π bond. The px orbital is in the x plane, while the dzx orbital is in both the z and x planes. They cannot overlap sideways.

(D) dxy + dyz: This combination cannot form a π bond. The dxy orbital is in the xy plane, while the dyz orbital is in the yz plane. They cannot overlap sideways.

So, the combinations that lead to the formation of a π bond are (A) py + py and (B) dyz + dyz.