Distinguish between a sigma and a pi bond.

Which of the following overlaps is incorrect (assuming $Z$-axis is the internuclear axis)?
$(A)$ $2p_y + 2p_y \to \pi$-bond formation
$(B)$ $2p_x + 2p_x \to \sigma$-bond formation
$(C)$ $3d_{xy} + 3d_{xy} \to \pi$-bond formation
$(D)$ $2s + 2p_y \to \pi$-bond formation
$(E)$ $3d_{xy} + 3d_{xy} \to \delta$-bond formation
$(F)$ $2p_z + 2p_z \to \sigma$-bond formation

$A$ molecule $(X)$ has $(i)$ four sigma bonds formed by the overlap of $sp^2$ and $s$ orbitals,(ii) one sigma bond formed by $sp^2$ and $sp^2$ orbitals,and (iii) one $\pi$ bond formed by $p_z$ and $p_z$ orbitals. Which of the following is $X$?

What is the overlapping of atomic orbitals? When is a covalent bond formed?

The correct order of the extent of overlapping is:

In an acetylene molecule,the bond between the carbon atoms consists of: