The $p-p$ orbital overlapping is present in the following molecule:

Among the following ions,the $p\pi - d\pi$ overlap could be present in:

$A$ 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 $X$?

Which of the following represents the most effective overlapping?

The strength of bonds formed by $2s-2s$,$2p-2p$ and $2p-2s$ overlap has the order

Select the molecule which has only one $\pi$-bond.